Hal 570
■
Example 17.2 CM Activated Sludge Process Effluent
A CM activated sludge process with recycle is being operated at the following conditions: Q = 1650 m 3/d (0.436 Mgal/d) r = 0.60
S 0 = 200 mg COD/L
Lab studies have shown that Eq. (17.1a) is the best kinetic model. The rate constants associated with the process are as follows: k = 0.95 mg COD/mg COD/mg VSS/d
K = = 85 mg COD/L
Y = = 0.48 mg VSS/mg COD
k e = 0.07 d-1
The average VSS concentration in the clarified effluent from the secondary clarifier has been found to be 15 mg/L; VSS in the influent in negligible. negligible. Determine
the
effluent
soluble
substrate
concentration,
total
effluent
COD
m = = 1650 d 8 h (241 dh) = 550 m V=0.436 ×10 8 h . = 19 43430 ft =
concentration, waste flow rate from the underflow of the secondary clarifier, sludge age, and MLVSS if Xvt is 5000 mg/L and
is 8 h.
The volue of the basin is
In U.S. units:
The substrate balance results in
There are two unknowns in this equation, S e and X and X v. Because r and X and X vr are known, a vr are biomass balance around the aeration basin will also provide information on Xv on Xv.. In – In – Out Out + Generation = 0
1 r + V=0 This expression and the substrate balance expression have the same two unknowns, Xv Xv and Se Se. this equation may be solved for Xv, Xv, which can then be substituted into the substrate balance expression.
= 1 = ( ) [1 ]
Making the substitution,
Halaman 571
This equation may be converted into a polynominal or solved by trial and error for Se. The polynominal expression is
1 1 1 = 0
Substituiting the values for the parameters and solving tha quadratic (only the positive root is meaningful) results in S e = 34 mg/L The total effluent COD, S Te., assuming VSS has a compotion of C 5H7 NO2 is found from Eq. (17.26), STe = 34 + 1.42(15) = 55 mg/L X v may now be determined from the substrate balance,
mg mg mg mg 200 34 85 34 = = = 0.0.L333 d0.L95 d− 34L mgL L = = 1 835835 mg/mg/LL 0.80534 .95 d−mg34 mgL 0.07d− = 0.0.060060 d− 1 = = = 0.4888534 L = 17.17.6 d
The sludge age is determined from the relation
The waste flow rate is determined from a meass balance around the system, which shows that the solids wasted are equal to the solids produced.
=( ) = = mg mg − 0. 4 8 0 . 9 5 d 1 835 34 = 85 mgL 34 mgLL L 0.07d−1 835 mgL ×550 m(1 000m L)(101kg mg)=60.8 kg/d mg mg − 0. 4 8 0 . 9 5d 1 835 34 L L = 85 mgL 34 mgL 0.07d−1 835 mgL ×19 430 m(28.ft3 L)(454 ×101 lb mg)=134 lb/d = kg 10 mg mg 1 m m (60. 8 )( )( ) 15 (1 650 ) 1 000 L L d kg d = mgL 15 mgL 5 000 =7.2 m/d 454 × 10 mg 1 gal mg gal l b 134 ( )( )15 (0. 4 36 × 10 ) 3. 7 9 L L d l b d = mgL 15 mgL 5 000 =1 908 gal/d
Solving this equation for Q w,
First r xV will be determined,
Halaman 572 In U.S. units:
Substituting r xV in the equation for Qw,
In U.S. units:
Qr is only 0.4% of Q.
17.7 PLUG FLOW ACTIVATED SLUDGE TREATMENT
If the same substrate removal expression applies in a PF and a CM reactor, the PF system will be more efficient. However, effluent quality may be similar between these two systems because of production of slowly degraded compounds (Skyses, 1983) and the safety factor applied in the design. Byproducts are formed as a result of substrate removal and growth and products of microbial maintenance and cell lysis (these phenomena are endogenous decay phenomena). The balance between byproduct formation between these two routes depends on the sludge age and the HRT in the process. If the exogenous substrate contained in the influent is largely being removed, then the effluent quality will mostly consist of relatively nondegradable byproducts; the rate of production of soluble organics is primarily controlled by biomass level, which is controlled by sludge age. The hydraulic flow regime and raw substrate removal kinetics would have only a secondary influence on effluent quality. Various models have been proposed (Hao, 1998; Skyes, 1982) for byproduct formation but there is not a large amount of agreement at present. Achievement of flow regimes near a PF condition is difficult because of the intense aeration and mixing in aerobic processes. From tracer tests Murphy and Boyko (1970) found that many aeration basins designed to be PF were actually nearer a CM flow regime. Baffles or separation of basin compartments are required to achieve PF conditions. Hal 573 PF conditions promote the growth of biomass that has good setting qualities (Chudoba et al, 1973) A PF systems provides the opportunity to tailor the oxygen input into the aertartion basin to correspond to the demand as the sewage travels through the aeration basin. The oxygen demand will be higher at the influent end of basin as readily degradable substrate are metabolized. At the effluent of basin the rate of oxygen uptake is lower because of the lower rate of endogenous metabolism and absence of readily degraded substance. Operation of a system where the oxygen input is decrease along the aeration basin is a process variation known as tapered aeration. Operation of conventional activated sludge systems in the taoered aeration mode has a concomitant benefit of improving the quality of the floc existing the aeration basin and lowering the TSS concentration in slarified effluent (Das et al, 1993). This result is not surprising considering the performance of flocuculation systems (Chapter 13), wherein decreasing velocity gradients produce the most highly flocculation effluent.
The PF configuration is shown in Fig. 17.1c. A PF basin will have recycled flow fron the underfow of the secondary clarifier returned to the e=head of the basin. The flow rate into the basin wil be Q’ = Q + rQ where Q is the flow rate into the system Qr is the flow rate into the aeration basin The substrate concentration at the beginning of the basin (S ) i depends on the reycle flow rate, which contain substrate at S e and the influent rate and substrate concentration S 0. A simple mixing equation describes S i. it is based on the mass balance at the confluence point of the influent and recycle lines.
= ++ = ++
(17.33)
The time required to achieve a given amount of substrate removal is determine by setting up a mass balance on an elemental volume in a PF reactor as outlined in Section 10.1.2. in a steady state process, for the elemental volume given in Fig. 10.3, In – Out + Generation = Accumulation
Substituing A
∆ ∆=0 ∆ ∆ ′ = for
(17.34)
and simplifying the mass balance,
The appropriate substrate removal kinetic expression (Ews. 17.1a=17/2b) is substituted for
and the equation is integrated. A chance of variable from x to time t is made
by replacing Q’/ A (velocity) with dx/dt and applying the chain rule. Using Eqs (17.1a) as the kinetic model:
Hal 574
′ = = =
In PF reactors, for kinetics expressions that incorporate X v (Eqs 17.1a or 17.2a), the average VSS concentration in the aeration basin,
, is used instead of X v . the concentration of
biomass increases at the front of basin as substrate is metabolized. In the later portion of the reactor, ehere substrate concentration is near exhaustion, VSS concentration will decrease because of endogenous decay. But the variation of concentration of VSS throughout the basin will not be sufficient to cause erroneous results in a typical ctivated sludge process (SRT >5 HRT) (Lawrence and McCarty, 1970) and the integration can be performed analytically. Separating the variables in the above expression and integrating it,
where
= ln =
(17.35)
is the substrate concentration at the beginning of the basin is the substrate concentration at the effluent end othe basin is the time for liquid to pass through the basin (
= V/Q’ )
Note that the time of treatment is different from the liquid HRT which is in Section 10.3). By substituting the relations for be related to the HRT and influent concentration,
= V/Q’ (as shown
and Q’ into Eq. (17.35) the treatment can .
The equation for sludge age and rate of recycle (Eqs. 17.28 and 17.32, respectively) are the same as developed for the CM process with recycle except that Xv. Also Eq (17.29) applied to a PF reactor will be yield
is substituted fir
instead of Xv. The substrate
balance expression cannot be replaced with the kinetic expression (
) because of variavle
substrate concentration in the aeration basin (see problem 24)
17.8 VARIATION OF THE ACTIVATED SLUDGE PROCESS
Variation in flow regime is a fundamental process embodiment many other odifications may be incorporated into an actvated sludge pocess.
17.8.1 Sequencing Batch Reactors
The operation of sequencing batch reactor (SBR) systems is dscribed in Fig. 17.2. SBR systems are hybrid systems wth some characteristics of continuously incoming wastewater. Aeration may be delayed or not be applied at all during the fill period to improve sludge setleability by favoring the growth of microorganism that form flocs (Section 17.12). During the react phase, although the reactor contents are mixed, the whole reactors contents remain in the tank for the spevified duration of this phase. This is an ideal PF condition for this phase. At the end of the react phase, aeration and mixing are terminated. Ideal quiescent settling, not subject to flow currents or other irregularities, occurs. In a SBR system the fill time (tf ) for an individual reactors depends on the available volume in the reactor. The available volume is the total volume of the reactor less Hal 575 The ortion occupied by the settled sludge remaining in the reactor after the previous cycle. In equation form:
= ∝ = = ∝
(17.36)
(17.37)
where
∝
Q is the influent flow rate during the fill period is an unoccupied fraction of the total volume of the batch reactor at the beginning of the fill period
is the volume of a batch reactor when it is empty is the available volume at the beginning of the fill period
One complete cycle is composed of the fill, react, settle, draw, and idle periods.
(17.38)
=
where the subscripts c, f, s, d, and I indicated cycle, fill, react, settle, draw, and idle, respectively
In a steady flow situation, each of the reactors will pass through the same cycle period and a quasi-steady condition will be achieved for the system. In a twi-tank SBR system, while one reactor fills, the other reactor passes through the other periods of the cycles; in a three-tank system, after a reactor fills, it has two fill times to pas through the remaining periods of a cycle. The equation expressing the general relation is
(17.39)
1 = ∓
where n is the number of reactors in the SBR system The cycle time and the durations of each period, in particular the fill time, and the available volume are the physical operating constraints on the system. Although there is flexibility in changing the duration of any phase, there are tr adeoffs in increasing the duration of one phase at the expense of the others. Minimum settling times must be observed or sludge will wash out of the system and deteriorate effluent quality as well as reduce the inventory of sludge in the reactor. The substrate concentration remaining at the end of the fill period is a variable function of the volume and the kinetic expression for substrate removal that applies during the fill period. The mass balance during the fill period is In – Out + Generation = Accumulation
0 = (17.40)
where
is the rate of substrate removal during the fill period
Using Eq. (17.2b) for
Hal 576
= ,Eq. (17.40) becomes
The equation can be further simplified by recognizing that dV/dt = Q. Using this, dividing by V, and rearranging the equation ,
=
(17.41)
Q/V is 1/t. At the beginning of the fill period, the reactor contains a volume V l and t = V /Q; l at the beginning of the fill period t = V B /Q. The differential equation to be solved is
= (17.42)
The equation is solved using an integrating factor. The solution of the equation is
= −∫ + ∫ −−∫+
where
C is an integrating constant
Performing the integration,
(17.43)
− = −
= −
Using the initial condition, t = V I / Q and S = S e :
= −/ ⇒=( )/ = −
Making the substitution for C, the final equation becomes
(17.44)
The substrate concentration at the end of the fill period can be found by using
t f = V I / Q
S = S t
− − = = (17.45)
For most of the substrate removal expression a numerical solution is required to solve the differential equation describing substrate removal during the fill period. Droste (1990) has evaluated physical and reaction constraints on SBR systems for various models and compared them to other activated sludge processes. Calculating the amount of removal during the react period is straightforward. The equation for substrate removal is intergrated directly as in the case for PF. Using Eq. (17.2b),
= ⇒ ∫ = ∫
or
= −
Hal 577 where S f is substrate concentration at the end of the fill period S e is the substrate concentration at the end of the react period
The amount of reaction time is found directly from Eq. (17.39) once the durations of the other periods are estabablished. The concentration of substrate at the end of the react period is the effluent concentration. Because the feast-famine feed cycle in a SBR process promotes the growth of microorganisms that settle well and there are no currents from inflow or outflow during the settle phrase, the amound of time for settling is likely to be substantially less than detention times used in conventional continuous flow secondary clarifiers. The draw time dpends on the design of the drawoff intake device and the disturbance caused as the water leaves the reactor. Many intake devices float. The solids do not need to be completely settled to begin drawoff of the clarified supernatant with this type of intake. The sludge age in SBR system is based on the principles discussed earlier. The effective sludge age depends on the time during which the biomass is active. Biomass activity occurs during the fill and react periods when food is available and other conditions favorable for metabolism exist. During the settle and draw phases, substrate should exhausted and oxygen supply is negligible.
Sludge age is calculated based on the avarege amount of solids in the SBR. The actual sludge residence time in system is:
(17.46)
=
Where Xv is the average concentration of sludge in the reactor.
is calculated by taking the
average of the amount of sludge resent at the beginning of the fill period divided by the total volume of the reactor and the concentration of sludge present at the end of
the react period.
(
w is
the amount of sludge wasted (in the clarifies effluent and the concentrated
sludge drawn off during the draw of idle period) of a daily basis. The average amount of solids will be slightly different from the value obtained using
this procedure because of the varying amount of sludge in the reactor during the react phase resulting from growth and endogenous decay and, also, solids loss in the clarified effluent during the draw phrase will cause the amount of solids in the reactor to vary during this phase. Howaver, these variations will be small and the procedure just given is a practical approximation to the average amount of biomass in the system. The effective sludge age depends on the fraction of the cycle time in the fill and draw periods. The actual sludge residence time is reduced by this fraction to obtain the effective sludge age.
(17.47)
= +
SBR systems are sometimes operated with an anoxic or anaerobic fill period. However, substrate is being supplied to the biomass during this period and the substrate will at least undergo the initial stages of the metabolism, where it is adsorbed and absorbed
by the biomass and some extracellular enzyme activity begins. Therefore, the fill period should be include in the effective sludge age when the process is operated in this manner.
Consistent definitions of sludge age have not been used in the literature. The reader is cautioned to check the definitions of sludge age used when making comparisons among different stidies.
17.8.2 Fixed Film Activated Sludge Process Incorporating fixed medium into the reactor provides a surface on which microorganisms can attach and grow. The microorganism on the medium surface are not washed out of the system with the liquid and higher sludge ages and biomass concentrations are obtained in the reactor. Submerged fixed media will be used with aeration supplied by diffused aerators. Mass balances for substrate and biomass are made in the normal manner. Hamoda (1989) studied a bench – scale system fed synthetic, sugar – based waste. Biomass washout was determined by plotting HRT versus SRT at different influent COD concentrations. SRT depends on attached and suspended biomass in the system. The slope of the best fit straight line for an ifluent COD concentration is the washout factor, F .
= ΔΔ
(17.43)
The washout factor was then plotted against influent COD concentration and a linear relation ( F = 2.2 x 10-4 S 0 ) was found.
17.8.3 Extended Aeration Activated sludge processes with long HRTs ( on the order of 24 h ) are referred to as “extended aeration” processes. A high sludge age is also maintaineed in the process. Extended aeration processs are typically designed to handle domestic waste from small communities. Often the operator will not be present an a continous basis. The amount of sludge generated from low flow rates does not generate enough sludge to be wasted on a regular basis. Also, the high sludge age gives rise to more endogenous decay of sludge. Sludge is allowed to accumulate in the system until the sludge blanket in the clarrifier rises to the point where excessive solids begin to washout with the clarified effluent. The time between sludge removals may be 6 months or more. The process is a continous flow process where the contents of the aeration basin are CM. The long aeration time ensures a high removal of degradable organics; however, the high sludge age
promotes the formation of pinpoint floc, which does not settle as well as zoogleal floc formed at lower sludge ages ( see Fig. 17.4 ). Consequently, effluent quality deteriorates somewhat because of the presence of unsettled fines. The secondary clarifier may be a separate unit from the aeration basin or an aeration clarifier unit ( Fig. 17.5 ) may be supplied. The clarifier is separated from the aeration basin by a soolid baffle wall as shown in the figure. The recirculation pattern causes the recycle off solids from the clarifier. The process operation provides a practical solution for wastewater treatment for small communities, schools, isolated residences ( such as hospitals or retirement complexes ), or other small operations where a full – time attendant is not warranted. Hal 579
F igure 17.5 Extended aeration system with internal clarifier.
A high sludge age and a long average hydraulic time enable the process to cope with more highly variable or intermittent flows from small operations and to maintain stable treatment. In many extended aeration plants the influent to the aeration basin is not settled in primary clarifier. Many commercial manufacturers supply extended aeration package plants.
17.8.4 Other Process Variations Other process variations have been utilized for various situations. The sstep aeration process depicited in Fig. 17.6 is a process modification that changes the environment in a PF reactor to an environment that would be observed in a CM reactor. By introducing the influent along several points in the reactor, more uniform environmental conditions are made to occur in the reactor. In particular, loading rates and oxygen demand rates are, respectively, approzimately the same at aany point in the reactor. Tthe flow regime remains PF in the ideal case.
If confentional process is operated with shorter detention times and higher F:M rations it is referred to as a modified aeration process. Removal efficiencies in this process are lower. However, if high mixed liquor suspended solids (MLSS) concentrations are maintainedwith low detention times, process performance improves. Tthis variation is known as a high rate process, F:M ratios are intermediate between the conventional and modified aeration process. The contact stabilization process (Fig. 17.7a) economizes the volume required for treatment. The influent is introduced into a small reactor or contact tank that also receives biomass in a starved condition. Soluble substrate is readily adsorbedand absorbed by the starved biomass. The mixed liquor leaving the contact reactor is settled and the biomass is concentrated. The biomass is then sent to an aeration basin.
F igure 17.6 Step aeration activated sludge process
Hal 580
F igure 17.7 (a) Contact stabilization process. (B) COD uptake in the contact tank.
Where is resides for a much longer time and metabolism and stabilization of the captured substrate occurs. The key to a successful operation is the residence time in the contact chaamber. After the substrate is taken up by the microorganisms, metabolism initiates the release of secondary byproducts and results inn a typical COD variation with time as shown in Fig. 17.7b. Over-or underdesign of the contact time will result in effluent quality deteriation. Contact basin retenttion times are in the range of 0.5-1 h (Metcalf and Eddy, 1991). Aeration times for the sludge in the stabilization basin are typically in the range of 3-6 h. Effluent quality from this process will not be as that achieved in conventional processes because of the nature of the interplay between uptake and releaseof COD in the contact phase and release of small amounts of COD from the biomass while it is in the clarifier. However the volume requirements of the process will be approximately 50% of a conventional process (Metcalf and Eddy, 1991). The deep sshaft activated sludge process (Fig. 17.8) is another variation that is used where costs of land are high. There are a number of installations in Japan. These reactors may extend 150 m (500ft) into the ground (Sandford and Chisolm,1997). A mixture of sewage and air travels down in downcomer and upward in a riser. The higher pressure achieved i the shaft improve oxygen transfer
rates and more oxygen can be dissolved with beneficial effects on substrate stabilization. DO levels can range 25 to 60 mg/L in the shaft. Microorgaanism metabolic rates are not affected by the high pressures. The nature of flow in the reactor is PF (Sandford and Chisolm,1997). Temperature is nearly constant throughtout the year in deep shaft reactor. Separation of the biomass from the mixed liquor is amenable to flotation (see Section 20.6.2). The effluent from the deep shaft reactor is supersaturated with air when exposed to atmospheric pressure. The release of dissolved air from the mixed liquor forms bubbles that float suspended solids to the surface. Hal 581
F igure 17.8 Deep shaft activated sludge process
Pure Oxygen Activated Sludge Process Pure oxygen activated sludge processes are conventional activated sludge configurations but relatively pure oxygen as opposed to air is supplied to the aeration basin. Larger plants (>40.0000 m3/d or 10.5 Mgal/d) generate the oxygen cryogenically whereas smaller plants use a selective sorption process, pressure swing adsorption. Because the gas flow rates are significantly reduced, supplemental mixing must supplied in the mixed liquor to keep solids in suspendion. The aeration basins are also covered to minimize the escape of the oxygen-rich gas above the basin; however, some gas must be exhausted to remove carbon dioxide produced in the process. Nevertheless, there will be an accumulation of CO2 in the mixed liquor, which results in a decrease of pH. Buffering may be required with alkalineagents, particulary if nitrification is desired. Higher DO concentrations will be maintained in the aeration basin compared to an air activated sludge process. The generally accepted minimum DO concentration for an activated sludge process is2.0 mg/L (?Parker and Merrill, 1976). At this concentration mass transfer of oxygen to the
flocs will not be limiting factor. Higher DO concentrations in a pure oxygen activated sludge process will have some influence on the flora that are established.
Powdered activated Carbon Activated Sludge Process Carbon may be added to a conventional activated sludge process to enhance treatment particulary with respect to removal or recalcitrant an toxic substances. The process is known as the PAC (or PACT) process because powdered activated carbon is used (Fig. 17.9). In addition to adsorbing various constituents, the carbon provides a support surface for microorganism. The adsorbed compounds are exposed to the biomass on the carbon surface for the sludge age as opposed to the HRT. Sludge production is higher in a PACT process because of carbon additives.
Design Parameters and Operating Conditions for Activated Sludge Pr ocesses Typical design parameters and operating conditions for various modifications oc activated sludge processes aree summarized in Table 17.3. The rangers given are suitable.
TABLE 17.3 Design Parameters and Operating Conditions for Activated Sludge Processes a
F:M Process
x ,
modification Conventional
5-15
Tapered aeration
Kg BOD5/kg d
MLVSS-d
MLTSS Loading rate 3
d ,
kg BOD5/m -d
0.2-0.4
0.3-0.6
4-8
5-15
0.2-0.4
0.3-0.6
Step aeration
5-15
0.2-0.4
Modifield aeration
0.2-0.5
Contact
5-15
mg/L h
r, 1500-3000
0.25-0.5
4-8
1500-3000
0.25-0.5
0.6.1.0
3-58
2000-3500
0.25-0.75
1.5-5.0
1.2-2.4
1.5-3
200-500
0.05-0.15
0.2-0.6
1.0-1.2
0.5-1.0 b
1000-3000 b
0.5-1.5
3-6e
4000-9000c
stabilization Extended aeration
20-30
0.05-0.15
0.1-0.4
18-36
1500-5000
0.5-1.5
High rate aeration
5-10
0.4-1.5
1.6-16
2-4
3000-6000
1-5
Pure Oxygen
3-10
0.25-1.0
1.6-3.3
1-3
3000-8000
0.25-0.5
a
rd
From Metcalf and Eddy (1991), Wastewater Engineering: Treatment, Disposal, Re use, 3 ed., G. Tchobanoglous and F. L. Burton, eds., McGraw-Hill,
Toronto, used with permission of McGraw-Hill,Inc. b
e
Contact unit
Solids stabilization unit.
F igure 17.9 Bench scale CM (left) and PAC (right) activated sludge systems.
For a wide variety of wastewaters; however, some industrial wastewaters may require longer HRTs.
17.9 SLUDGE PRODUCTION IN ACTIVATED SLUDGE SYSTEMS
The sludge produced in an activated sludge process or any biological treatment process is a functionof the substrate characteristics, sludge, age, and other environmental conditions, particularly temperature of the mixed liquor. The rate of production of biological solids is given by Eqs.(17.4) or (17.27). Note that these equations describe the total production of biological solids, which appear in the effluent from the secondary clarifier and in the waste sludge line from the underflow of the secondary clarifier. An observed yield factor (a net yield factor as discussed in Section 17.3.2) can be formulated for a process at given operating conditions. P Xb = (-Yr s - k e Xv) = -Y obs.b r sV
(17.49)
Where P Xb is rate of biological sludge production Y obs.b is the observed yield factor for biological solids Sludge age affects the concentration of MLVSS and temnperature and other environmental factors affect both the yield factor and the endogenous decay coefficient.
The rate of total solids production from a biological treatment process will be higher than the rate calculated from Eq. (17.49) because this equation only describes biological VSS production and there is normally a significant amount of solids in the influent to the aeration basin. Degradation of influent solids has been discussed in Section 17.6.2. The rate of production of solids from a biological process is
= + = os.
(17.50)
Hal 584
Where
is rate of sludge production
f b is the ratio of VSS : TSS for biological solids f t is the fraction of influent TSS that was not degraded Y obs: is the observed yield factor for total solids Studies are needed to determine the factor f t in Eq. (17.50). It is a function of the degradable portion of the influent VSS : TSS ratio for the influent. An example will illustrate the factors on which f t depends
Example 17.3 Sludge Production in an Activated Sludge Process
An activated sludge process is treating primary settled effluent that has a BOD 5 of 150 mg/L and TSS concentration of 90 mg/L in a flow of 25000 m 3/d (6.61 Mgal/d). The detention time in the aeration basins in 5 h. MLVSS is 2100 mg/L and the effluent endogenous decay coefficient is0.05 d -1. The VSS : TSS ratios for biolofgical solids and influent SS are 0.80 and 0.50, respectively. Find the observed biological yield factor and the range of the observed yield factor for total solids. Also find the possible range for r ate of total solids production.
⁄ 24 h = = 15020mgL ( 5 ℎ d )= 624mgL⁄ /d m = =25000 d 5 h(241 dh)=5208 m =5.21 10L
The rate of substrate removal (Eq. 17.15) is
The volume of the aeration basins is
Applying Eq. (17.49),
− ⁄ ⁄ 0. 6 8 624 mg L/d0. 0 5 d 2100 mgL . = = 624mg⁄ /d = 0.512 mg VSS/mg BOD 5
The rate of biological solids production is
=.= (0.512 mgmgBODVSS)624 mg BODd ⁄L5.21 10L(101kgmg)
= 1.67 x 103 kg/d
If none of the influent VSS were degradable then f 1i = 1 in Eq. (17.50) and the maximum theoretical solids production would occur.
⁄ 24ℎ . = = = 0.0.58120mgmgmgVSS⁄⁄ 5190 ( ℎ624 /⁄ ) = 0.640 mg TSS/mg BOD 5 + 0.692 mg TSS/mg BOD 5 = 1.332 mg TSS/mg BOD 5
=.= (1.332 mgmgBODVSS)624 mg BODd ⁄L5.21 10L(101kgmg)
= 4.33 x 103 kg/d
Hza; 585 The other extreme for degradation of the influent VSS is that all influent were degradable suspended substrate that were transformed into biological solids. To solve the equation it is also necessary to know the contribution of these degradable VSS to the influent BOD5. Commensurate with the assumption that all VSS were metabolized in the process it will be assumed that the influent VSS fully contributed to the influent BOD5. The influent VSS concentration is 0.50(90 mg/L) = 45 mg/L. It is also necessary to know the VSS : TSS ratio of the degradable VSS. It will not be 0.50 and a value of 0.75 will be used (the ratio o degradable VSS to TSS in the influent is not
necessarily the same as the ratio od VSS : TSS for biological solids). Now the degradable VSS is associated with (45 mg/L)/0.75 = 60 mg TSS/L. Besides the 15 mg/L of suspended inorganic solids associated with the biomass there would be 30 mg/L ISS in the influent. Some of influent ISS could be solubilizes in the process but it will be assumed that no ISS has been solubilizes. The factor f t in Eq. (17.50) can now be determined as
= 3090 =0.333
The VSS component of influent TSS is implicitly contained in the influent BOD5. The inorganic solids associated with the influent degreadable VSS ae assumed to be released with metabolism but the formation of biomass from metabolism of the degradable VSS will reassimilate inorganics. The observed yield factor and rate of solids production are
os, = os, =0.640 mgmgBODTSS 0.3533h96240 mgdmg/LTSS/L (24dh) os, =0.640 mgmgBODTSS 0.231 mgmgBODTSS =0.871 mg TSS/ mg BOD X =os,=(0.871 mgmgBODTSS)(624 mgd BOD/L) 5.21 ×10L (101kg mg) os, =2.83 ×10 kg/d 2.83 ×10 4.33 ×10 The actual solids production rate will lie between
and
kg/d.
there are numerous factors to be determined in arriving at the actual rate of solids production. Observed yield factors can be calculated from pilot studies and with sufficient measurements the values of the factors can be assessed. Their values are of academic interest because the overall yield factor is known. But this example illustrates the phenomena that are involved.
Observed yield factors for domestics wastewater eith primarysedimentation range from 0.33 to 0.8 mg TSS/mg BOD5 ; without primary sedimentation the range is 0.62-1.18 mg TSS/mg BOD5 for sludge ages up tp 30 d and temperatures between 10 and 30oC (WEF and ASCE, 1992a). Processes that operate at.high sludge ages such as extended aeration have lower observed yield factors than processes operated at lower sludge ages. Higher temperatures result is lower observed yield factors than lower temperatures.
