Page 792 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Biological Reactors 747

Another way often advocated to increase digester perform- 3. Calculate SRT(2),
ance is to have a longer solids-retention time, SRT. This is V(reactor)
defined as SRT 1 ¼
Q(solids) 1
3
3
¼ 2839 m =110:6m =day
mass of solids in digestor ¼ 26 day (Ex23:3:2)
(23:48)
SRT
mass of solids leaving per day
Discussion
The longer SRT results in higher VSS reduction. The data
Ordinarily, u ¼ SRT. However, if the mass of solids in the
are from the Fort Collins WWTP.
digester is increased by increasing the concentration of solids
flowing into and maintained in the digester (through better
23.4.4 OPERATION AND MONITORING
thickening) then the SRT is increased correspondingly and
SRT u. Expressing Equation 23.48 mathematically, An issue that is often given only an afterthought level of
attention in design is for the draining of all tanks with provi-
V(reactor) [VSS] sions for cleaning. This is true especially of anaerobic tanks
(23:49)
SRT ¼ where access to the bottom of the tank is, as a rule, not
Q(solids) [VSS]
adequate. Another issue is safety, since explosions have
been known around anaerobic digesters due to the methane
or
gas. As a rule, the gas is collected and used for heating the
digester. Methane is a greenhouse gas and should not be
V(reactor)
(23:50)
SRT ¼ vented to the atmosphere; combustion (flaming) will convert
Q(solids)
the gas to carbon dioxide and water.
Since, for a given waste loading on a plant, the mass-solids flux 23.4.4.1 Process Upsets
from the digester is constant, Q(solids) [VSS] ¼ constant. Close operator control is necessary in operating the anaerobic
Therefore, if Q(solids) is reduced due to improved thickening process. The most likely problem is the acid production reac-
before the digester, and since V(reactor) does not change, then tion getting ahead of the methane fermentation reaction and
SRT is increased accordingly. Example 23.3 illustrates the causing an acid condition, and an upset. Toxic inputs are
application of the concept. another potential problem.
23.4.4.2 Indicators and Tests
Example 23.3 Effect of Sludge Thickening on SRT Monitoring is essential if any impending upsets are to be
discovered and avoided. Change is the key to detecting upsets.
Given Some of the parameters monitored include pH, volatile acids,
3
[SS] o ¼ 4.75% [ ¼ 47.5 kg=m ] volatile suspended solids (in and out), and gas production.
3
3
[VSS] o ¼ 0.80[SS] o [¼0.80 47.5kgSS=m ¼ 38.0kgVSS=m ] Since pH often changes after the fact, it has little use in
3
3
Q(solids) ¼ 40,000 gal=day [¼151.4 m =day ¼ 6.31 m =h] operation. Volatile acids are recommended by some. However,
V(reactor) ¼ 750,000 gal ¼ 2,839 m 3 gas production is the most sensitive indicator of an upset and is
the one which is prominently mentioned in the list of indicators
Required available. This is quite logical since CH 4 and CO 2 are the final
If the solids inflow is increased to [SS] o ¼ 6.5% [ ¼ 65 end products of the methane reaction. The composition of the
3
kg=m ], calculate the new SRT.
gas should also be measured periodically. Ordinarily this is
Solution about 70% methane and 30% carbon dioxide. Since carbon
1. SRT(1), dioxide is also an end product of the acid-formation reaction,
an increase in CO 2 levels also indicates an impending upset.
V(reactor)
Measurements of volatile suspended solids in the incoming
SRT 1 ¼
Q(solids) 1
sludge, in the digester and leaving the digester, are essential
3
3
¼ 2839 m =151:4m =day so that the loading on the digester can be ascertained and the
¼ 19 day (EX23.3.1) volatile solids reduction can be determined. Temperature and
alkalinity should also be monitored.
2. Calculate Q(solids) 2 ,
23.4.4.3 Percent Reduction of Volatile Suspended
Mass flow solids ¼ Q(solids) 1 V(SS) 1
Solids
3
3
¼ [151.4 m =day] [47.5 kg=m ]
A common measure of effectiveness of the anaerobic diges-
¼ 7192 kg solids=day
tion process is ‘‘percent reduction’’ in volatile suspended
Mass flow solids ¼ Q(solids) 2 V(SS) 2
solids (VSS). Figure 23.15 illustrates the method to calculate
3
7192 kg solids=d ¼ Q(solids) 2 [65 kg=m ] the percent reduction in VSS with calculations shown in
3
Q(solids) 2 ¼ 110.6 m =day Example 23.4.