A

National Diploma in Drinking Water Assessment March 2002 Tutor Notes and Home Assignment for Unit Standard 18452

Describe drinking water treatment filtration

processes, and management of critical points

Otago :Polytechnic

Otago Polytechnic Drinking Water Assessors Course

US 18452 - Filtration

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CONTENTS 1 1.1 1.2 1.3 1.4 1.5 1.6 2 2.1 2.1.1 2.2.1 2.2 2.2.1 2.2.2 2.2.3 2.3 2.4 2.5 2.5.1 2.5.2 2.5.3 2.6 2.6.1 2.6.2 2.6.3 2.7 2.8 2.9 2.9.1 2.9.2 2.9.3 2.9.4 2.9.4 2.9.5 2.10 3 3.1 3.2 3.3 3.4 3.5 3.6

INTRODUCTION:.........................................................................................................4 Purpose of Assignment: ........................................... ................................................................... ............................................... ........................... .... 4 New Zealand Scene: ............................................. .................................................................... .............................................. ................................ ......... 4 The Purpose of Filtration ........................................... .................................................................. .............................................. ......................... .. 4 The Principles of Filtration .............................................. ..................................................................... ..........................................4 ...................4 Waters Suitable for Filtration.................................. Filtration.......................................................... ................................................ .......................... .. 5 Solids Removal ........................................... .................................................................. .............................................. ..........................................5 ...................5 RAPID SAND FILTRATION ............................................... ...................................................................... ........................................6 .................6 Rapid Sand Filters......................................... Filters................................................................ .............................................. ....................................... ................6 6 Open Gravity Filters ............................................ ................................................................... .............................................. ............................ ..... 6 Pressure Filters: .............................................. ...................................................................... ................................................ ................................ ........7 7 Rapid Sand Filtration Mechanisms....................................... Mechanisms............................................................... .................................... ............7 7 Mechanical Particle Removal .............................................. ..................................................................... ................................... ............7 7 Centrifugal Effects.................................... Effects........................................................... .............................................. ....................................... ................8 8 Settlement Sett lement ............................................. .................................................................... .............................................. ...........................................9 ....................9 Rapid Sand Media..................................... Media............................................................ .............................................. ...........................................9 ....................9 Rapid Sand Filter Headloss.................................. Headloss......................................................... .............................................. ............................. ...... 11 Filtration Operating Modes ............................................... ...................................................................... ...................................... ...............12 12 Rising Head Filtration: .............................................. ...................................................................... ...........................................1 ...................12 2 Declining (flow) Rate Filtration ............................................. ..................................................................... .............................. ......12 12 Parallel Filtration .............................................. ..................................................................... .............................................. ............................. ......12 12 Media Sizing and Depth.......................................... Depth................................................................. .............................................. ........................... .... 13 Media Size ............................................ ................................................................... .............................................. .........................................1 ..................13 3 Breakthrough Index ............................................. .................................................................... .............................................. .......................... ... 14 Media Depth ............................................. .................................................................... .............................................. ..................................... ..............15 15 Mixed Media .............................................. ...................................................................... ................................................ ........................................ ................15 15 Filter Backwashing................................. Backwashing........................................................ .............................................. ............................................16 .....................16 Filter Monitoring.................... Monitoring........................................... .............................................. .............................................. ..................................... ..............18 18 Turbidity Turbid ity ........................................... .................................................................. .............................................. .............................................18 ......................18 Flow......................................................................................................................19 Headloss ........................................... .................................................................. .............................................. .............................................19 ......................19 Particle Counters ................................................ ....................................................................... ............................................... ........................... ... 19 Headloss Distribution ................................................ ........................................................................ ...........................................2 ...................20 0 Microscope Use ........................................... .................................................................. ............................................... .................................. ..........20 20 Filter Operating Problems ............................................. ..................................................................... ..........................................21 ..................21 MEMBRANE FILTRATION................ FILTRATION....................................... .............................................. .............................................. ......................... 23 Membrane Processes........................................ Processes................................................................ ................................................ ................................. ......... 23 Membrane Materials and Operation ............................................... ......................................................................23 .......................23 Membrane Cleaning...................... Cleaning............................................. .............................................. .............................................. .............................. ....... 24 Membrane Integrity Checks ............................................... ...................................................................... ..................................... ..............24 24 Nano Filtration .............................................. ..................................................................... .............................................. ..................................... ..............25 25 Reverse Osmosis filtration........................... filtration................................................... ................................................ ..................................... .............25 25


4 4.1 4.2 4.3 4.4 4.5 4.6 4.7 5


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OTHER FILTRATION METHODS................... METHODS .......................................... ............................................. ................................ ..........26 26 Slow Sand filtration...................................... filtration.............................................................. ............................................... ..................................... ..............26 26 Micro-straining..........................................................................................................26 Diatomaceous Earth Filters............................. Filters.................................................... .............................................. .................................. ...........27 27 Ion Exchange Filters ............................................ ................................................................... .............................................. .............................. ....... 28 Bag Filters ............................................... ....................................................................... ................................................ ...........................................28 ...................28 Granular Activated Carbon ............................................ ................................................................... .........................................29 ..................29 Cartridge Filters.................................... Filters............................................................ ................................................ ............................................30 ....................30 OPERATING ISSUES ........................................... .................................................................. .............................................. ............................... ........ 31

Assignment:...................................................... Assignment:............................... .............................................. .............................................. .............................................. ......................... .. 33



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Unit Standard 18452 - Filtration 1

INTRODUCTION:

1.1

Purpose of Assignment:

This unit standard sets out to detail the various processes used to filter drinking water to remove undesirable components. There are a diverse numbers of filters and the range will be considered on a filter type by type basis.

1.2

New Zealand Scene:

Virtually all New Zealand drinking water supplies use one of three types of filter - ground filtration, rapid sand filtration (assisted by chemical coagulation) or unfiltered. There are a limited number of bag filters, cartridge filters, membrane filter plants and diatomaceous earth filters. There are also two slow sand filters (Palmerston and Little River). These less common filters will also be considered but in less detail.

1.3

The Purpose of Filtration

Filtration is the process of removing suspended solids from a water flow. It does not (strictly speaking) include the removal of dissolved matter. The chemist's definition of dissolved is the material that will pass through a 0.45 micron filter. Filtration is the removal of suspended (i.e. not dissolved) material. These notes will include the processes of ion-exchange, slow sand and activated carbon filtration which are effective at lowering dissolved matter. The purpose of all of these filters is the removal of unwanted material from water to improve its potability.

1.4

The Principles of Filtration

The apparent method by which filters remove suspended material is straining - the particles are held back by having holes smaller than the particles removed. However, secondary mechanisms are also important. These include filter "ripening" - a build up of larger particles on the surface of the filter that decreases the aperture size allowing smaller particles to be removed. This is especially important in slow sand filters.



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Another key secondary process is the settling of particles onto the horizontal parts of the filter media. This is particularly important in rapid sand filters. This settlement process is enhanced by the curvature of the flow around the media, spinning the particles out to the edge of the flow through the pores due to their higher density. There are also electrostatic forces which attract particles to the media. This occurs to a limited extent in rapid sand filters and is very significant in ion exchange filters.

1.5

Waters Suitable for Filtration

Not all water is suitable s uitable for filtration filt ration without pre-treatment. pre-t reatment. Turbid water with very fine clays, cl ays, coloured water with almost dissolved colloidal matter and water with pollutants in solution all require some chemical assistance to filter properly. Whilst membrane filtration will remove virtually everything from water if the holes size is small enough, even these filter types usually require chemical assistance to remove colloids in the micro-filtration size range.

1.6

Solids Removal

The filtration process results in an accumulation of the removed solids in or on the surface of the filter. The length of a filter run - i.e. the time before these solids must be flushed from the filter media - depends on the incoming solids concentration. For this reason, many filters are preceded by a clarification step to lower the solids loading on the filter. This applies to all filter types. Without this clarification step, the filtration process is called "direct filtration". This mode is suitable for low turbidity filtration -usually where there is good raw water storage to reduce the solids. Filter washing is almost always done used filtered water (called the "filtrate"). If the incoming solids level is too high, the filtrate used to wash the filters may represent the entire flow resulting in no filtrate available for consumption. So the water to be filtered should not be too dirty. A reasonable rule of thumb for rapid sand filters is that if the clarified water turbidity is over 100 NTU, the total production will probably be needed ne eded for filter washing. The exception to this is suction type membranes which are washed using air pulses.


2


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RAPID SAND FILTRATION

Rapid sand filters are the most common filter type and can produce excellent quality drinking water. For this reason, these will be examined in considerable detail.

2.1

Rapid Sand Filters

2.1.1

Open Gravity Filters

Most rapid sand filters operate as open gravity filters - i.e. they are open to the air and use gravity to overcome the frictional headloss through the sand media. A typical sand filter has the following layout:

Water in Pool level Inlet trough unfiltered water Sand media Filter Nozzles on floor Filtrate Out Underdrains

The inlet trough may be across one end of the filter, along the side or suspended along the centre. It is designed to minimise "jets" of water hitting the sand surface, drilling holes into it.

•

•

The pool level slowly rises as the sand clogs and the headloss builds up.

The sand media is usually about 600 mm deep. It is sometimes underlain with a coarser layer (large sand or gravel).

•

The filter nozzles are spaced at about 200 mm centres both ways. The are usually plastic "domes" or similar. similar . They connect into the pipe pi pe underdrains under. und er.

•



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The underdrain system is a series of pipe laterals connected to a (bigger) pipe across one end. This leads the filtered water away.

•

The above diagram represents the standard "open gravity" filter. There are several other rapid sand configurations:

A Cut-away View of a Filter in a Series of Filters 2.2.1

Pressure Filters:

Pressure filters are similar except the entire filter is inside a pressure vessel - either a cylinder on its side or, more often, a cylinder on end. Pressure filters are often used where a pumping circuit is in place - very common in swimming pools systems - or where the plant is well below the hydrostatic head level. Horizontal filters are used at Fielding, vertical cylinders at Inglewood.

2.2

Rapid Sand Filtration Mechanisms

As noted above, the filtration mechanisms include both primary and secondary effects.

2.2.1

Mechanical Particle Removal

Mechanical action is simply having a smaller holes than the particle being removed. For spheres, the holes between the particles are about 30% of the sphere diameter:

1,000 micron spheres 300 micron holes


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Standard filter sand ranges from say 600 microns to 1,200 microns so the holes range around the 200 micron size. This hole size is far bigger than a protozoan cyst (at 3 - 8 microns) or silt particles at say 1 micron. micr on. Clays are even smaller. Mechanical filtration will thus hold up floc particles but not individual small particles. As the holes are partially clogged, the filter "ripens" and smaller particles are trapped mechanically on the particle layer developed. This results in "enhanced filtration" and is why there is a preference to divert the first part of the filtered water to waste as this will water contain more particulates than later in the filter run. This clogging may develop simply on the media surface - known as filter blinding or it may develop into the media depth. This latter is preferred as it gives more media volume to hold solids - and hence longer filter runs. Conceptual filter entry system for in depth filtration Flow direction

Media block

In practise, most filter clogging occurs within the first 100 mm of media depth.

2.2.2

Centrifugal Effects

The centrifugal effect is due to the higher density of the particle than that of the water it is in. The centrifugal acceleration is due to the radius of the bend that the flow is turning around. The actual equation is A = v2 / r and this equation gives an insight into how this mechanism is so effective. As the turning radius is small, so the acceleration can be significant even at the low flow velocities in a filter. This acceleration pushes the particles to the outside of the flow, increasing their chance to land on a settlement area and be retained.

Sand grains Particle "spun out" into quiescent area and deposited ☻

Flow


2.2.3


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Settlement

Settlement is one of the primary ways in which smaller particles are captured and held within a sand filter. In a 600 mm deep layer of sand comprised of sand grains averaging 0.9 mm in diameter, there are about 800 layers of sand (due to some vertical overlap). Given that the flow spends most of its time going sideways to skirt around a grain, the effective surface area is high due to this large number of layers. These give an overflow rate of 2 l/sec/m2 divided by 800 layers of 1 m2 each or 0.009 m/hour. This rate is significantly lower than the settlement rate in a clarifier of about 2 m/hour. Using this theory, a particle with a specific gravity of 1.3 would settle in a sand filter if the diameter (of the particle) was 4.5 microns or more. Obviously, this means that many small particles are removed re moved by settlement.

2.3

Rapid Sand Media

Conventional rapid sand media is sand - hardly surprising given the process name. However, other media forms are used for particular reasons. Sand is specified by: • • • • • • •

Mineral substrate - usually silica (also known as quartz) Minimum size - all particles are retained on a particular sieve Maximum size - all particles pass through a particular sieve aperture size d60 - the diameter which 60% (by mass) of the particles are smaller than d10 - the diameter which 10% (by mass) of the particles are smaller than Uniformity coefficient - the ration of d60/d10 Grain shape - usually sub-angular or rounded

Expanding on these one by one: Mineral substrate - silica is usually specified as it is durable and will last without losses due to abrasion. Silica is also very common as sand so this is not expensive to specify. Other options include garnet (which is denser) but this is significantly more expensive. Impurities that are not welcome are pumice (even though it is silica, it wears down), chalk (it wears down) and organic material (such as twigs, shell fragments etc.). The most common sand in New Zealand is Waikato River sand - a poor sand comprising a variety of rock fragments, not well sieved, an excess of fine material but cheap in the 1970's. It also contains some pumice which breaks down in size giving excess fines. Minimum/Maximum size. These are often 1:2 - e.g. 600/1,200 microns or 425/850 microns. These size is also often specified as sieve numbers. The rough rule of thumb is sieve number times microns equals about 16,000. Standard sieves used in water filters are:


Sieve number

50 40 25 20 14


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Mesh (and grain) size microns 300 425 600 850 1,200

Uniformity Coefficient - Uc = d 60/d10 . This gives a measure of how well spread out the size range is. If a sample of (identical) golf balls were sieved, 60% would pass a 50 mm sieve as would 10%. The Uc is therefore 1.0000. A well graded (i.e. well spread out) sample will give a bigger Uc than a sample that is more consistent. Normal practice is to seek a Uc less than 1.5.

If the two samples drawn on the graph below are compared: •

The sample with the S-shaped curve gives

Uc = 810 / 425

= 1.91.

•

The straight line sample gives

Uc = 920 / 640

= 1.44.

This means the S shaped curve will be a denser mix than the other, make a better sand for mixing concrete but is a worse filter sand - there is less void space in it. Note that the analysis analy sis curve is plotted with a log lo g scale on the X axis.

Sieve Sieve Analysis sis 100.0%

n a 80.0% h t s 60.0% s e l t 40.0% n e c r e 20.0% P 0.0%

100

10000

size - microns

Grain shape also affects the bulk density of the media as well as the headloss of the flow going through the sand. Rounded or sub-angular grains do not pack together as tightly as


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angular grain shapes. Hence they have more pore volume and are generally preferred over angular. Sharp fragments have about 50% more headloss than spherical grains. The Waikato River media suffers in this respect as well as some grains in it are very angular.

2.4

Rapid Sand Filter Headloss

The flow through the sand bed inevitably results in friction and an associated headloss. The amount of this headloss may be assessed using the Carman Kozney equation: Hl = f x . L . x (1- n) x v2 (s x d) n3 g where:

Hl = headloss - in metres of water head f = friction factor (dimensionless)

= 1.75 + 150 x (1 - n) R

R =

Reynolds Number (dimensionless)

= sxvxd υ

υ

2

= kinematic viscosity of water - m /sec

= 1.15 x 10-6 @ 15º C

L = sand depth - in metres s = particle shape factor (no units)

= 1 for spheres = 0.89 for rounded sand = 0.73 for angular sand = 0.65 for crushed glass

The shape factor "s" has been established empirically. d = d60 of the sand media - in metres n = porosity of the media - no units - the "percentage" that is voids (calculated from the overall density and the grain density). Typical value = 0.4 v = overall flow - metres/second

= flow / area

g = gravitational acceleration - metres/second squared - = 9.81 m/s2 This equation is used in the design of sand filters. The key thing to note is that the porosity is very important - the headloss varies as this cubed. So a dense sand with n = 0.40 has one third more headloss than a loose sand with n = 0.37. The headloss through a filter starts off uniformly spread through the media. As the filter clogs with solids, the headloss becomes concentrated near the top of the filter. Monitoring work done at Hamilton showed up very clearly that this headloss accumulation occurs in the top 100 - 200 mm of the filter with different types of (poorly graded) media.


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This can lead to negative pressures further down the filter bed which will cause air bubbles to form. The air comes out of solution under negative pressures. These air bubbles prevent water flow giving higher headlosses again - this phenomena is called "air blinding".

Water surface

Static pressure (no flow) Clean filter head Middle of filter run head End of run head Sand Negative pressure pressur e Air blinding area Gravel

-0.4

-0.2

0

0.2

0.4 0.6 0.8 Headloss thru sand and gravel

1.0

Underdrain Area 1.2 1.4 1.6

1.8

Diagram of Head Distribution causing Air Blinding 2.5

Filtration Operating Modes

Given that the headloss builds up with time (as the media clogs), there are a number of operating choices.

2.5.1

Rising Head Filtration:

The first, and most common option, is to keep the flow constant and allow the filter pool to slowly rise. This is called "rising head" and is useful in that the operator (or some control equipment) can see how dirty the filter is becoming. When the headloss reaches the set maximum, the flow into the filter is stopped and the backwash cycle begins.

2.5.2

Declining (flow) Rate Filtration

The second is to keep the filter pool depth constant and let the flow slowly decrease as the filter clogs. This requires a flow meter on each filter to see how much flow is going through each filter and to detect the (low) flow at which the filter should be backwashed.

2.5.3

Parallel Filtration

A third option is to mix these two ideas by having all the filters sharing the same inlet pressure, or pool and sharing the flow between bet ween them t hem according acc ording to how h ow dirty dir ty they th ey are a re relative re lative to each other. The (common) pool depth rises until a pre-set figure. When this headloss is reached, the next filter is washed. By washing the filters in the same sequence, the next to be



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washed should be the dirtiest as it has had the longest run time. This option is called "parallel rising head, declining rate". In general terms, a filter will process about 50% more flow at the start of its run than at the end. This means that declining rate will give about 25% more flow overall than rising head. The third option - parallel filtration can lift total filter capacity by about 35% overall over rising head.

Headloss

Flow

Run time Rising Head Mode (Constant flow thru filter) 2.6

Media Sizing and Depth

2.6.1

Media Size

Run time Declining Rate Mode (Constant head loss thru filter)

The choice of media size(s) and density(s) is set by two factors. The first is the desired filter performance and what is being filtered out. Finer media gives better filtration but uses up more headloss and has less storage for solids - so it needs backwashing more often. oft en. The second consideration is the backwashing cycle. Bigger media needs higher backwash flows to clean it. Warmer water needs higher flow rates as well. The following chart shows how important these factors are: (These flows will fully lift and fluidise the sand)


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Sand Filter Backwash Rates for Sand with Gs = 2.67 c 30 e s / m25 m - 20 e t a R15 h s 10 a w k 5 c a B 0

0C 6C 12 C 18 C 24 C

0.5

1

1.5

Sand size - d60 - mm

Chart showing minimum upwash rates to fully fluidise and wash sand filter media (based on Leva's Formula) 2.6.2

Breakthrough Breakthrough Index

To assist in filter design and analysis, a U.S. researcher named Hudson devised a "Breakthrough Index" (BI) based on filter media size, bed depth and headloss. These factors are calculated together in the following formula: 3

BI = V x d x Hl L

where BI = Breakthrough Index (implicit units) V = filtration rate - cm/minute d = sand diameter - d10 - mm Hl = headloss at end of run - metres L = bed depth - metres By entering this data into the equation, the Breakthrough Index for the filter is calculated. The higher this BI is, the more stress is applied to the floc particles on their way through the filter. Hudson then classified floc that could take these stresses on a matching scale as follows: Very Strong Medium Light Weak

BI = 16 BI = 8 BI = 4 BI = 2



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This gives a design tool for deciding on media size based on floc strength. The floc strength numbers are empirical and based on the assessors experience - i.e. they are relative to other flocs encountered. If the filter BI is higher than the assessed floc strength, then the flocs will be sheared in the filter. This will result in particle release and filter breakthrough. The final water turbidity will rise. 2.6.3

Media Depth

The depth of media is also a design variable. The industry practice is to allow about 1,000 times the diameter of the smallest media particle as the bed depth. So 14/25 sand - the most common - is 600 to 1,200 microns and the normal bed depth is 600 millimetres. Deeper beds do not give longer run times - the filters clog from the top - and give more headloss and construction cost. Shallower bed depths lower the protection against particle escapes. There has been US research on final water turbidity versus bed depth. This showed negligible gains in (lower) turbidity over bed depth/media size ratios over around 600. The figure of 1,000 is thus a conservative figure. 2.7

Mixed Media

In an effort to improve solids penetration into the media, mixed media are often used. The immediate difficulty with different particles sizes is maintaining the desired sequence of particle size in the bed. If small and large sand are mixed, the larger size will fall to the bottom of the bed first as the media settles after a fter a backwash. back wash. This leaves le aves the finer sand on top - exactly the wrong order for better solids penetration. To overcome this sorting issue, the use of media of different densities is adopted. At the conclusion of a backwash, the upflow velocity slowly reduces down to zero (then reverses for normal filtration). The media particles start to settle as the velocity decreases to their settlement velocity - the velocity shown on the backwash chart above. The concept of using denser small media is that smaller sized particles will settle to the bottom due to their higher hi gher density before befo re the larger (less dense) dens e) particles start to settle. se ttle. The most common combination used is anthracite coal over sand. The coal density is about 1.3 T/m3 compared to 2.67 T/m3 for silica sand. In water, this difference is further accentuated by the buoyancy effect e ffect so the coal becomes beco mes 0.3 compared to the sand san d 1.67. So a sand grain at 0.8 mm diameter with Gs = 2.67, has the same fall velocity as a coal grain (Gs = 1.3) of 1.9 mm. So the mix would be 0.8 mm sand with coal of 1.8 mm and the coal should settle out on top. This leaves the coarser material on top, allowing the big particles to get caught by the coal and the small particles to go through the coal and be caught on the sand.



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The range of useful media is limited by the need for significant density differences together with the cost of and durability of the material. Apart from anthracite coal, garnet is used as it has a density of around 2.9 - not a lot heavier than silica but a little. Mixed media beds do not fall into neat, separate layers but mix as the different materials are not all the same diameter. Thus the big coal will fall faster than the small sand. A New Zealand effort in this area is the "activated silica sponge" material from the Waikato River source near Hamilton. This material has modified pumice in it to yield a larger, lower density top layer. There are also ionic charge sites on the pumice which add to the filter performance. This material has had some success but it is not a universal panacea and it displays many of the problems of normal sand.

2.8

Filter Backwashing

As noted above, all filters require backwashing when the pores becomes blocked with the solids removed. Backwash must last long enough to remove most of the solids in the media. Backwashing can be initiated by any one or more of the following factors: •

Time - many plants wash filters on a regular schedule of say every 2 or 3 days.

Turbidity - if the turbidity from a filter starts to rise, this indicates that solids are being pushed through thr ough the media and it needs nee ds to be backwashed - immediately. immedi ately.

•

Headloss - as the headloss rises, so does the probability of floc shear and particle breakthrough. Reaching Reach ing a pre-set headloss level l evel will initiate a backwash. back wash.

•

Flow - some plants set the amount of water to be filtered between backwashes. The flowmeter is totalised and when say 1,500 m3 has been filtered, the backwash sequence begins. This is useful to allow for flow rate changes.

•

Backwashing sequences can follow one of the three main concepts: British - this sees an air scour followed by a medium rate water backwash. The air is applied at a rate of 18 - 36 m/hour for about 2 to 3 minutes. The concept is to break up the filter structure and rub the grains against each other to loosen polyelectrolytes. The water is applied at rates too low to lift or expand the bed and thus this method often results in mudballs, filter cracking and other cleanliness issues. Typical backwash flow rates are around 5 - 6 mm/second instead of about twice this needed to lift the bed. The water lasts about 5 - 8 minutes - long enough to run clear. American - this method does not use any air but uses high water flow rates - around 35 - 50 m/hour for 4 - 5 minutes. These lift the bed and the expansion is about 20%. The process is very good at eliminating mud balls but is less effective at removing polyelectrolyte. Media stratification - fines ending up on top - can result and shorter filter run times follow on from this.



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French - this is usually the most effective of all three techniques and uses air and water at the same time. The sequence starts with air at 50 - 60 m/hour and water at 10 - 15 m/hour concurrently. This wash lasts for about 5 - 10 minutes. It is followed by water alone for about 5 minutes at around 15 - 25 m/hour - too low for full bed fluidisation but enough to remove any trapped air.

A modification of the American technique is "Automatic Valveless Gravity Filtration" AVGF. This technique allows the headloss build up to prime a backwash siphon. When this priming is complete, c omplete, the backwash b ackwash siphon starts and cleans the filter using a high h igh water flow fl ow rate. No air is used as this would break the siphon. This method has the advantage of not needing any site storage of backwash water or a backwash pump. It has the disadvantage of tending to trip the whole bank of filters into backwash at once.

Empty backwash siphon

Head loss

In

AVG during filter run . Note Head loss is less than the height of the siphon tube. The filtered water collects in the top tank

Top tank - open to the air - overflows to filtered water outlet Filtered water out Bottom tank - contains media - is a pressure vessel Filtered water pipe going up to top chamber

Full backwash siphon

Head loss

AVG during backwash. The siphon has primed and is pulling the top tank full of water back through (up) the sand. The incoming unfiltered water is also discharged to waste.

In No filtered water out - the top to p tank has stopped overflowing ov erflowing Sand has expanded upwards under backwash



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Empty backwash siphon

Headloss

AVG at start of filtering cycle . Note siphon has broken and the top tank is re-filling before filtered water is produced

In

AVG filters often have the following features: Air-pipe interlock between filters so that if one is in backwash, the others will not prime. A siphon priming venturi where the initial flow through a venturi sucks the air out of the top of the siphon. On banks of filters, the inlet flow is shut, diverting the flow to the remaining filters. Filter banks can be run in parallel rising head, declining rate with primed, sequenced washing. Surface scour of the media at the start of backwash. This is achieved by a rotating arm with high (underwater) jets playing onto the media surface. Priming by a vacuum air pump to allow positive control of backwash initiation.

• •

• •

•

•

AVG filters backwash typically around 14 mm/sec at the start of backwash, reducing to around 9 - 10 mm/sec at the end of the flush. This results in good washing but high polyelectrolyte polyelectroly te use will cause mud-balling mud-ballin g due to the lack of an air wash was h phase. AVG filters are fairly common in New Zealand - about 50% of the plants constructed in the 1970's have AVG filters. 2.9

Filter Monitoring

2.9.1

Turbidity

Rapid sand filters are usually monitored by checking the output turbidity on each filter. This can be done with separate turbidimeters or by switching the sample drawn into a single meter from filter to filter. Note turbidity meters take time to settle down and switching should be at more than 15 minute intervals. Turbidity should be 0.05 NTU or lower if the process is performing excellently. Turbidimeters work by passing a beam of light through the water sample and measuring how much of the light is lost by particle reflection and/or absorption. These instruments come from a variety of manufacturers and have some configurations options. Most have a reference beam to allow for different optical effects. The basic concept is to send a beam through the sample and pick up light at both "straight ahead" and at 90 degrees. The higher the turbidity, the more the light is scattered, reducing the ratio between the two receivers.


2.9.2


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Flow

Flows per filter should be monitored individually. The Portals plants have a weir to measure filtered flow discharged - these are not very accurate and care should be taken in using these figures. Typical flow figures are about 2 l/sec per square metre (i.e. mm/sec) of filter area. Declining rate filters will start higher - about 3 mm/sec. Some multi media filters will run higher than this.

2.9.3

Headloss

When measuring headloss through a filter, it is important to isolate the various components. Headlosses occur through the media and the underdrains - the latter should be identified. Typical headloss through the sand should be kept to under 1.2 metres for a 600 mm deep bed. At this headloss, the BI is: 3

BI = V x d10 x Hl L = 12 x 0.653 x 1.2/0.6 = 7 - average floc.

For headlosses higher than this, better floc is needed or it will be pushed through the media.

2.9.4

Particle Counters

Particle counters register the number of particles in different size ranges. They work by one of three different methods: • • •

Electrical sensing - not in common use for water treatment Light blockage - most suitable for particles over 1 µm (1 micron) Light scattering - most suitable for particles under 1 µm

The basic principles of operation of the light (or optical) methods are similar to turbidimeter principles. (Most of the t he technology has been be en transferred by the t he manufacturers.) The most usual light beam wavelength is 790 nanometres which is in the laser category. The light scattering models cost about twice as much as the light blockage models. The industry standard low range is 2 µm as this is the limit for realistic sensor cleaning systems. Particle counting experience in New Zealand is still relatively limited. However, some issues are becoming clear: •

The numbers of particles per unit volume is significantly high. The maximum achievable removal rates when a rapid sand filter is well managed still give counts around 20 particles per ml - 20 x 106 per m3! A well designed and operated plant will



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normally have about 100 particles/ml. These figures are for particles in the > 2 µm range. •

•

•

The size distribution of cryptosporidium oocysts as reported by a particle counter shows about 90% are between 2 and 3 µm. This does not stand up well against the N.Z. DWS requirement of 3µm as the smallest size to be monitored (DWS Sec 3.2.3.1 - first bullet point.) One of the reasons for this difference is that the reported minimum size of the oocysts (3 µm) was as estimated using a microscope. The particle counter recognises it as a smaller object as the oocysts are partially transparent thus appearing smaller when measured using light as the technique. The size distribution of particles in filtered water is almost invariably a function of the diameter cubed. This simple fact is the easiest way to check whether an instrument is in calibration. If the 2 µm count is (10/2)3 - 125 times the 10 µm count, then the instrument is almost certainly performing correctly. To lift the count high enough to check this, simply wiggle the sample tube to send down a wave of loose particles.

Field tests with particle counters often show a variety of other factors predominate over filter run time. Diurnal fluctuations (due to algae cells?) and other events (filter loading due to others backwashing etc) may be of significance. Often, there is no strong correlation between elevated particle counts and turbidity.

Particle counters will become more important as monitoring tools as the industry learns how best to use them and what wha t their limitations are. are .

2.9.4

Headloss Distribution

This check on where the headloss is building up can be field checked using a length of thin walled tube, 50 - 100 mm in dia. Push this into the media and note the difference in water level (or pressure) between the pool and the depth at which the tube is sitting. Note that the sand does not have to be displaced - the measurement is of the static water pressure at that depth in the sand. This very simple test will show how well the floc is penetrating into the filter media.

2.9.5

Microscope Use

A low power (20x - 40X) microscope will provide good information on the condition of the media on a number of fronts. It will show grain material, shape and cleanliness. It is a very useful way to check on the efficacy of caustic soaking and similar cleansing treatment.


2.10


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Filter Operating Problems

These fall into two categories - mechanical damage and cleanliness. Mechanical damage can occur, particularly in the underdrain area. This may include broken or blocked nozzles and pipework. The simplest check on reasonably good conditions in the underdrain area is the distribution of air during air scour which should be even throughout the filter bed. This is a visual inspection - actual measurements are feasible but difficult - and will show any "dead areas" very quickly. These areas are likely to have either broken pipework or blocked nozzles or a breakage has allowed either the sand or some support suppor t media (the (t he gravel) down and into the pipes. There is usually no "quick fix" for these problems - the filter media usually has to be removed, by hand, to examine the problem area. A similar inspection should be made during a water backwash. Here, it is more difficult to see low flows from one nozzle if the wash rate is good. However, many filters have poor upflow rates and a missing nozzle flow will show up. The bed should expand visibly to avoid the problems noted n oted below. belo w. If there th ere are individual "jets" appearing app earing at the sand surface, surrounded s urrounded by areas of static sand, sa nd, the backwash rate is too low. Poor backwash flow rates lead to a steady build up of mud and polyelectrolyte within the media. The result is that parts of the media become blocked, with little or no filtration occurring in these areas and high filtration rates in the others. The media can become sorted by size with coarser coarse r sand in the clean jet areas. ar eas. Mudballs develop as part of this build up. A common cause of this binding up of the filter media is excessive poly usage. Polyelectrolyte is, by design, a "sticky" material and is difficult to dislodge by water flow alone. Air is virtually essential to remove poly and avoid a steady build up on the media. The air works by agitating the grains against each other - scraping the poly loose, ready to be carried away by the water. The excessive use is often driven by a desire to reach lower turbidity levels - a good objective but high poly doses are not the preferred method. Better all round chemical dosing and filter maintenance is a better method of achieving low turbidity. Care with valve operation is also important. Filter cracking will also occur. The sand becomes continuous, rather than free and loose and the block will bind together. Large and deep cracks form both in the middle of the filter and around the edges (against the walls). These allow short circuiting of the flow and provide less protection for the water. wa ter. The best long term solution to filter cracking is to increase the backwash rate significantly. The wash rate should expand the bed up to 10% deeper - without losing media into the launder troughs. The troughs must be high enough to avoid these losses. The best short term solution is to clean the filter media chemically. Normally, caustic soda at about 0.5 - 1.0 % solution strength will dissolve the polyelectrolyte. The preferred chemical



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(caustic soda, chlorine or an acid) can be simply assessed using samples in the laboratory and a visual check under the microscope. It is also useful to sample and sieve the media on an annual basis. This will show whether the coal is being broken down (or lost in backwash) and how much fine material is accumulating in the filter. Less commonly, manganese, iron or carbonate may build up within a filter. These are more likely to show up as a colour change apart from increased headloss and/or reduced filter runs. All of these are more likely to be removed by acid than caustic. For any chemical soak treatment, use the air to stir it in and check where the waste water is to go. It must not be discharged to natural water under the Resource Management Act (and a clear conscience) - it will almost certainly be very damaging to the environment. Caustic and acid solutions can be neutralised in the filter, once again using air to mix the chemicals. Caustic soaking of silica media will impart a negative charge to it, assisting to hold any spare coagulant that may otherwise pass as a residual.


3


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MEMBRANE FILTRATION

Reference : U.S. EPA. Report "Low Pressure Membrane Filtration for Pathogen Removal: Application, Implementation and Regulatory Issues. (the EPA Report).

3.1

Membrane Processes

The EPA report sets out the classification of membrane filtration in Section 2.2. There are four classes - in order of increasing filtration size. The size quoted is the range of the largest particle that will be passed p assed through an intact int act membrane:

• •

•

•

Reverse Osmosis - RO - 0.0001 - 0.0010 µm - able to desalinate brine Nano-filtration - NF - 0.0009 - .0004 µm able to remove most salts, sa lts, viruses, colloids, colloid s, clays and most organics Ultra-filtration - UF - 0.005 - 0.15 µm - able to remove bacteria, protozoan cysts and silts Micro-filtration - MF - 0.1 - 8 µm - able to remove algae, some silt, bacteria and cysts and all sands

All of these processes are available in New Zealand but not all are at Water Treatment plants for public supply. MF plants are in operation at Oropi Road (a Memtec MF plant), Carterton (a MF bag filter) and Paraparaumu (a cartridge MF). Plants are proposed at Nelson and Mercer (both Zenon MF) and in Dunedin (undecided). As the pore size comes down, the differential pressure to drive the flow through the membrane increases. This is the primary reason that all plants are not RO plants. MF requires around 3 - 10 m of water head. This figure is significantly lower than early units which required much higher headlosses. UF operates under similar pressure losses but has a higher capital cost. The EPA report points out that 87% of the USA installed plants are MF with the balance being UF. Nano-filtration Nano-filtration - NF - is becoming more popular in Europe according to recent visitors. The primary advantage of NF over MF is the ability to remove coloured organics (colloids) without the need for preliminary coagulation. Coagulation is required at times for most MF plants that receive surface water with colloidal colour. The most common coagulant is ferric chloride as alum is not compatble with many membrane materials. Alum is used at Oropi Road, ferric chloride is planned for Nelson.

3.2

Membrane Materials and Operation

The various material are set out in the EPA Report - Sec 2.2 as are the different flow configurations.



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The most fundamental difference is whether the process is a suction one (such as the Zenon plants) or a pressure one (such as the Memtec plant). This difference is important when assessing how clean the incoming (feed) water is - the suction units are less prone to difficulties in dirty feed water. The key costing difference is the "flux rate" - how many l/sec per square meter of membrane. This determines how many modules and hence the capital cost of the plant. The EPA Report sets out the various configurations available. Both suction and pressure micro-filtration plants in use in New Zealand are composed of membrane tubes, about 1 mm in diameter and about 2 m long. These tubes (or filaments) are bundled in "elements" with about 20 - 40 elements bundled between a common inlet/outlet header (at one end) with the other end either closed off or used for a cleaning header. This unit (of elements between a pair of headers) is often called a module. The modules are run in parallel to provide the capacity required. Whilst different manufacturers use different configurations, all essentially allow hydraulic isolation of one module at a time for integrity testing. 3.3

Membrane Cleaning

All filters that are performing a real task build up solids on or in the filter and membrane units are no exception. Cleaning cycles involve reversing the flow with or without chemicals to aid the cleaning process. The cleaning cycles are usually initiated by headloss (or flow drop off for fixed headloss) due to filter clogging. Typical run times are over 12 hours. Many units have say hourly flow reversals accompanied by say daily chemical rinses. At say half yearly intervals, a thorough chemical clean may be performed. The chemicals involved tend to be reasonably strong solutions of acids and/or bases. Most require care in handling as a waste water.

3.4

Membrane Integrity Checks

All membrane units require membrane integrity checks to detect broken fibres or seal. These are usually "pressure and hold" tests to detect leaks, often using air to make the leak location under water much easier. This type of test is classified as a "Direct Method" in Section 4 of the EPA Report. Section 4.2 sets out the various direct test techniques. Sec. 4.3 of the Report sets out the Indirect methods - essentially monitoring final water quality - noting the significant confidence issues. The Paraparaumu plant often detects particles which appear to be shed by the membrane rather than passing through it. It would be illadvised to rely upon particle counting as a primary quality tool. Note the comments on the use of particle monitors in Section 4.3.2.


3.5


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Nano Filtration

NF removes organic compounds in the 300 to 1,000 molecular weight range, rejecting selected salts (typically divalent), and passing more water at lower pressure operations than RO systems. NF economically softens water without the pollution of salt-regenerated systems and provides unique organic desalting capabilities.

3.6

Reverse Osmosis filtration

RO can meet most water standards with a single-pass system and the highest standards with a double-pass system. RO rejects 99.9+% of viruses, bacteria and pyrogens. Pressure, in the order of 140 to 700 metres of head, is the driving force of the RO purification process. It is much more energy efficient compared to heat-driven purification (distillation) and more efficient than the strong chemicals required for ion exchange.


4


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OTHER FILTRATION METHODS

This section will outline (relatively brief notes) other filtration methods which are not in common use for Drinking Water in New Zealand. 4.1

Slow Sand filtration

Slow sand filtration utilises smaller (than rapid sand) sized sand media beds to filter the water at a very low loading rate. They are consequently significantly bigger in area. The real key to slow sand filtration is the development of a biological layer on the sand surface. The layer is called the "Schmutzdecke" and develops over the first few weeks of operation. It comprises of algae, protozoa, bacteria and viruses and acts as a barrier to most micro-organisms. Slow sand filters have been in use for over a century and are still used for some major supplies today. The Ivry sur Seine plant in Paris uses slow sand (along with several other treatment stages) as part of its barriers to pathogens and chemical pollutants. A typical slow sand filter has a bed depth of about 0.6 - 1.2 metres of fine (150 - 300 micron typical beach sand size) sand with a through-flow of about 2.5 m/day. The initial headloss may be around 0.7 - 1 m and this increases slowly as the Schmutzdecke layer develops and the bed clogs. The filters are not used for production until they have ripened (at least 3 days) and are kept in service until the head becomes too great - often a total pool depth of around 1.5 m. At this stage, they are taken out of service and the top 50 mm of media removed. Slow Sand filters remove significant numbers of pathogens. Giardia cyst removals have been reported at between 50 - 80% with similar but slightly lower removals of cryptosporidium oocysts. Water discharged form the slow sand filter is often low in oxygen and can have some carbon dioxide in it from the microbial activity. It is usual to aerate the flow. Slow sand filters are prone to excessive algae growth on the surface causing short filter runs. One method of reducing these is to screen out sunlight - at least partially. Algae will add taste and odour compounds and increase ammonia levels as they die, adding to the chlorine demand significantly. 4.2

Micro-straining

Micro-strainers are revolving drum screens utilising the finest woven metal mesh available. This mesh size is as low as 20 microns. This size was regarded as adequate to remove "lumps" too big for chlorine to penetrate and thus, coupled with chlorination, a process of ensuring proper disinfection.



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Micro-strainers were installed at many sites in New Zealand in the 1960's - virtually all have been replaced. There is still one plant operating in Dunedin (soon to be replaced with a membrane plant) and several in private plants such as meat works. They are too coarse as a filter to comply with filtration standards for protozoan cysts and do not remove colloids. A micro-strainer is typically a cylinder, with mesh walls, lying on its side. Raw water is let into the inside of the cylinder (which has otherwise closed ends) and it flows out through the mesh into the tank it is all placed in. The cylinder is about three-quarters immersed in water. The solids retained by the mesh are on the inside of the revolving cylinder. As the mesh is lifted out at the top of the circle, high pressure sprays from the outside are directed onto the mesh. The solids are washed off and collected into a trough, mounted just above the inside water level. The difference in the inside and outside levels is around 40 - 60 mm - the so called straining head. Wash water use is around 1.5 - 2.5% of flow through the plant. No chemicals are used and the wash water can usually be returned directly to a stream. The mesh is stainless steel. The manufacturer of New Zealand units was Glenfield and Kennedy of UK. 4.3

Diatomaceous Earth Filters

Diatomaceous earth (or kieselguhr) filters are pre-coat filters that can remove very small particulate matter, including some bacteria. They are practical only for limited volume applications but are common for swimming pools, beverage plants, and small installations. The principle of operation of these filters is to introduce diatomaceous earth (DE) particles to the incoming flow. These particles are the remains of diatoms, found in natural deposits. The size range of DE particles is from 5 - 100 microns. The flow is filtered through "septums" - fixed (usually stainless steel) vertical plates with fine holes on both sides. These holes are often covered with a cloth liner. The DE builds up on the septums, forming a very fine filter for the flow. There is obviously a pre-coat initial period when the flow is recycled. Once the pre-coat is established , the level of DE added to the flow is very low. The filtered water is used until the headloss through the filter becomes too high - often 3 -4 days. At this point, the unit is taken tak en out of production productio n and the cloth/DE removed to waste. was te. The head loss needed may be supplied through pressure (i.e. higher pressure on the outside of the filter layer) or vacuum - low pressure applied to the inside of the septum. DE filters are more expensive to operate than rapid sand filters for larger flows. This is primarily due to the high h igh labour content of o f the filter renewal.


4.4


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Ion Exchange Filters

Ion exchange filters have limited use in drinking water plants due to the high cost of the chemicals and the labour to apply them. Their most common use for drinking water is to soften waters for small plants. They are also used occasionally to remove nitrate, iron and manganese - again in limited volumes. An ion exchange system consists of a tank containing small beads of synthetic resin. The beads are treated to selectively adsorb either cations or anions and exchange certain ions based on their relative activity compared to the resin. This process of ion exchange will continue until all available exchange sites are filled, at which point the resin is exhausted and must be regenerated by suitable chemicals - usually salt. The concept behind ion exchange softening is to exchange calcium and magnesium ions with sodium ions. This softens the water as the sodium salts do not form hardness. The most common material to use as a sodium supplier and catalyst for this exchange is zeolite which is a naturally occurring mineral - a complex alumino-silicate. Certain green sands also act in this way. The general equation is Ca++ + Na2Z → 2Na+ + CaZ The calcium remains bound into the zeolite material whilst the sodium ion remains in solution. The resulting water is high in sodium which is normally not a problem. Eventually, all of the sodium ions in the zeolite are exchanged and the zeolite needs regeneration. This is done using salt solution - brine - as this is a very cheap source of sodium. CaZ + 2NaCl → 2CaCl + Na2Z The wastes from regeneration (calcium and magnesium chlorides) are usually discharged to the sewer. Similar equations occur for the removal of iron and manganese. 4.5

Bag Filters

These filters are a cylinder lined with a finely woven bag which acts as a membrane. They are limited in flow volume as the cylinder is under pressure from the headloss across the membrane. The flow is from the centre of the cylinder, out through the bag and into collection channels around and supporting the bag. Apart from several small supplies, there are some units are in use at hospitals and some food processors. They will stop gross solids but not no t material finer than about abo ut 1 micron.


4.6


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Granular Activated Carbon

Granular Activated Carbon (GAC) is a relatively common filtration method overseas. It is usually an adsorption rather than filtration process. It is also the primary method of treatment in "Water coolers" and similar point of use filters. The primary effect of these filters is to remove chlorine (and its compounds) thus improving the taste of the water. These filters must be changed periodically to avoid bacterial growth and are not easily reactivated in the field. Activated carbon is defined as a family of carbonaceous substances that are characterised primarily by their th eir surface s urface area, pore po re size distribution, and adsorptive a dsorptive and catalytic ca talytic properties. Different raw materials and manufacturing processes produce final products with different adsorption characteristics. Carbon in various forms has been used for the treatment of water and as detoxifying agent in medicine for many centuries. There has been an uninterrupted use of carbonaceous adsorbents since biblical times. Most GAC now in use is from wood carbon, coconut husks or, less common, bone carbon. During the twentieth century, GAC and powdered activated carbon (PAC) have been used to control taste and odours in drinking water. During the past 20 years, research on the use of adsorbents to treat drinking water has emphasised the removal of specific organics. The removal of organic compounds from drinking water has been based primarily on the measurement of organic matter as measured by the total organic carbon (TOC). Over 700 volatile organic compounds have been identified in drinking water. However, these compounds make up only a small fraction of the total organic matter in water. Approximately 90% of the volatile organic compounds that can be analysed by gas chromatography have been identified, but this represents no more than 10% by weight of the total organic material. Only 5%-10% of the non-volatile organic compounds that comprise the remaining 90% of the total organic matter have been identified. The EPA has categorised the organic compounds in drinking water into five different classes, each with distinctly different characteristics characteristic s of concern to those involved in water treatment. Class 1: organic compounds that cause taste and odour and/or colour problems. Class II: synthetic organic chemicals that are present in source waters from upstream discharges or runoff. Class III: organic compounds (precursors) that react rea ct with disinfectants to produce produ ce “disinfectants by-products”. Class IV: organic chemicals that are the disinfectants by-products themselves. Class V: natural organic compounds of little direct toxicological importance.

• •

•

• •

Today there are GAC beds in U.S. water treatment plants for removal of Class I compounds. Consideration is being given to the use of GAC for removal of Class II, III, and IV



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compounds as data become available. Class V compounds are of interest because they may compete for adsorption sites, thereby lessening the removal of other compounds. In New Zealand, PAC is added at several plants for the control of taste and odour. There are no GAC filters as such. Apart from adsorbing taste and odour compounds, GAC will destroy residual ozone. It is used for both of these purposes at a plant in Sydney. There, the GAC filters are in towers with the water trickled down through the media to discharge underneath. The tower is aerated by natural draught. GAC filters may also be set up very similarly to rapid sand filters with similar loadings.

4.7

Cartridge Filters

Cartridge filters can be described two general ways: •

•

Depth cartridge filters . In a depth cartridge filter, the water flows from the outside inwards through the thick wall of the filter where the particles are trapped throughout the complex openings in the media. The filter may be constructed of cotton, cellulose, synthetic yarns or "blown" micro-fibres such as polypropylene. The best depth filters have lower density on the outside and progressively higher density toward the inside wall. The effect of this "graded density" is to trap coarser particles toward the outside of the wall and the finer particles toward the inner wall. Depth cartridge filters are usually disposable, cost-effective, and are in the particle range of 1 to 100 microns. Generally, they are not an absolute method of purification since a small amount of particles within the micron range may pass into the filtrate. Surface filtration –pleated cartridge filters. Pleated cartridge filters typically act as absolute particle filters, using a flat sheet media, either a membrane or specially treated non-woven material, to trap particles. The media is pleated to increase useable surface area. Pleated membrane filters serve well as sub-micron particle or bacteria filters in the 0.1 to 1.0 micron range. More recent cartridges also perform in the ultra-filtration range: 0.005 to 0.15 micron.

Note that cartridge filters are often sold in "nominal size" ranges which is generally a lower value than their 99% removal size. This area is under study by a MoH led group and some standards are likely to be promulgated in 2002 or 2003.


5


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OPERATING ISSUES

In most plants, the filters are the primary barrier against protozoan cysts. Bacteria and some viruses will get disinfected but Giardia and Cryptosporidium will probably not be. Several significant incidents of disease have been caused by short term filtration lapses - the Milwaukee cryptosporidium incident is a well documented case. This incident saw a turbidity spike due to a chemical dosing control lapse - the turbidity was not very high and the spike lasted a short time. However, there were a lot of deaths and even more people ill - and suing the water supplier. The usual causes for poor rapid sand filter performance go back to the chemical dosing and clarification steps. The wrong chemical dosing regime (particularly pH) will give poor floc which may go through the filters - or not capture fine turbidity which will also pass through the filters. The causes of these problems are addressed in the notes for US 18450 - coagulation and flocculation. Filters may also present poor final water due to problems within the filter itself. Many of these problems result from filter cleanliness - particularly filter backwashing. Most plants in New Zealand have backwash flows that are about half of what is needed to lift and clean the sand properly. There are two serious risks that result from poorly washed sand filters. The first is a steady binding of say 80% of the sand leaving coarse sand "tubes" above the nozzles. These tubes take virtually all of the flow eventually - at five times the design flow rate and with coarse (rather than fine) sand. The result is a complete degeneration of the original filter design specification.

The way to identify this problem is by observation of the sand bed during backwashing. The filter may need to be washed at least twice to see the sand surface through the water - the first wash will probably be too dirty to see the sand surface through. These piping" areas will show a jet of sand mobilised and flowing upwards through them. Most of the sand will not be moved and bed vertical expansion will be negligible The second is the binding and subsequent cracking of the filter bed. The cracks will penetrate deep into the bed, allowing both high flows and a reduced bed depth for the filtration to occur in. The cracks show up clearly under clean water as black lines, often up to 20 mm wide and up to a metre or more in length. Cracks also are common at the side walls of the filter.

The short term solution to either problem is to caustic soak the sand to remove most of the accumulated polyelectrolyte, unstick the grains and try to wash the mud out of the filter. A compressed air lance may be useful before and possibly during this backwash to lift and break up mud balls.



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The longer term solution - higher backwash flows - is more difficult to implement. Often, there is a backwash pump that is too small and will need replacing to provide enough flow. In these cases, combining air with the water flow may be very useful. Another significant risk is building up the pressure in the underdrains during higher flow rates. This may rupture the underdrain floor system. Most of these are pipes buried in concrete and the high pressure in the pipes causes expansion, splitting the concrete in tension. There are several reported incidents of "blowing the floor up" - an expensive exercise as the floors cost over $1,000 per square metre. Poor quality water may also be due to hydraulic "shocks" - sudden changes in head or flow due to valve operation in the set of filters. These shocks may bump material off the media, resulting in a turbidity "spike". Rapid sand filters need time (normally around 20 - 40 minutes) to settle and "ripen" to produce their best b est quality qualit y water. This is the basis of filtering filterin g to waste (or, recycle) at the start of a filter run. Filter backwash water is recycled in some plants. Whilst this has water saving advantages, care must be taken that the contaminants removed do not accumulate in the recycled water and hence the filter. Membrane plants are also subject to the risk of too much reverse cleaning cycle pressure. Filament rupture - or the seals at the end on the filaments mounting heads - will provide a short circuit route for particle contamination of the final water. Summary:

If a filter is producing higher turbidities than is desired or specified, check the following processes: 1. The chemical dosing regime 2. The clarification unit (if any) 3. The cleanliness of the sand 4. The backwash regime 5. The membrane integrity test frequency A rapid sand filter should be able to consistently produce water below 0.1 NTU. If it is not, then one or more of the above processes is not functioning correctly. A membrane filter should also produce water that has a low turbidity. The actual turbidity will depend on several factors including chemical dosing regimes. However, less than 0.1 NTU is a sensible target for these plants as well.



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Assignment:

1

A filter sand is sieved with the following results. Plot the size distribution as % by weight (Y axis) versus size smaller than (X axis, log scale). Use this graph to calculate the d10 and d60 figures and the Uniformity coefficient. Comment on the answers and use the d60 figure to calculate the backwash rate required at 12° C. Sieve size Pan 300 microns 450 " 600 " 850 " 1,200 " 1,500 "

Weight retained (grams) 42.2 58.6 128.9 232.8 246.0 53.3 nil

2

A filter is to be used on average strength floc (BI = 8). The media is 14/25 sand, 700 mm deep with a flow rate of 6.8 m/hour. Calculate (using Hudson's formula) the maximum headloss allowable to avoid floc shear.

3

What is "multi-media" made of and why is it used?

4

If the filter in Q(2) is 3.6 m by 6.8 m in plan, how much water (in m3) would be used to backwash under the 3 methods described in Sec. 2.8 of the notes. Use mid-range figures for all rates and times.

5

A Portals plant has a backwash rate of 5.5 mm/sec, as measured on site during a backwash. The sand is the sand from Q(1) above. Comment on likely operating problems.

6

Use the EPA membrane manual to comment on the physical removal of pathogens and algae from drinking water.

A

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