Q O O

Figure 7.36 Effect of SRT on the steady-state concentration of various constituents in the last reactor of the Bardenpho system depicted in Figure 7.35. For comparison, the dashed curves represent the performance of the MLE system described in Figure 7.29. Influent flow = 1000 m'/day. Influent concentrations are given in Table 6.6. Biomass recycle flow = 500 m'/day; mixed liquor recirculation flow = 2000 m'/day, reactor volumes: V, = 50 m'; V, = 100 m'; V, = 75 m'; V4 = 25 m\ Parameters are listed in Table 6.3. The dissolved oxygen concentration was zero in the first and third (anoxic) reactors and 2.0 mg/L in the second and fourth (aerobic) reactors.

volume distribution among the various bioreactors, and recirculation ratio. However, it was not the intent here to minimize effluent nitrogen concentrations; rather, the point was to show the effect of adding additional anoxic and aerobic bioreactors.

A significant thing to note in Figure 7.36 is that the growth pattern of the autotrophic bacteria is different in the two bioreactor systems, even though the aerobic SRTs are the same. This emphasizes the point made earlier that although the concept of aerobic SRT is important to understanding the fate of nitrifying bacteria in systems containing anoxic zones, it is not the sole factor influencing their growth. System configuration is also important. This suggests that pilot scale studies coupled with system simulation are required to arrive at sound designs for such complex systems. In Chapter 11, we see how the results from such studies have been combined with full scale plant experience to allow development of design guidelines for biological nutrient removal systems.

Oxygen Requirement

Oxygen Requirement

Figure 7.37 Effect of SRT on the total steady-state oxygen requirement, nitrate utilization rate, and solids wastage rate for the Bardenpho system depicted in Figure 7.35 operating under the conditions listed in Figure 7.36. For comparison, the dashed curves represent the performance of the MLE system described in Figure 7.29.

Figure 7.37 Effect of SRT on the total steady-state oxygen requirement, nitrate utilization rate, and solids wastage rate for the Bardenpho system depicted in Figure 7.35 operating under the conditions listed in Figure 7.36. For comparison, the dashed curves represent the performance of the MLE system described in Figure 7.29.

7.7 BIOLOGICAL PHOSPHORUS REMOVAL PROCESS 7.7.1 Description

As discussed in Section 2.4.6, certain bacteria, known collectively as phosphate accumulating organisms (PAOs), have the interesting characteristic of concentrating phosphate in Poly-P granules when they are cycled between aerobic and anaerobic conditions. The Poly-P acts as an energy reserve that allows the bacteria to rapidly take up acetate under anaerobic conditions, storing it as PHB and other polyhydrox-yalkanoic acids (PFLAs). Soluble phosphate is released in the process. The PHAs, in turn, provide energy for growth under aerobic conditions. They also allow soluble phosphate to be taken up and stored as Poly-P. The difference in energetics between aerobic and anaerobic metabolism is such that more phosphate can be taken up than was released, providing a mechanism for concentrating phosphate within the biomass, allowing it to be removed via solids wastage.

The history of the development of biological phosphorus removal processes is one of the most fascinating ones in environmental engineering, beginning with ob servations of unexplained phosphorus removal in full scale CAS systems, arguments over the reasons for that removal, simultaneous development of processes by various groups, and conflicts over patent claims and infringements. Unfortunately, space does not permit a review of that history here, but the reader is urged to consult Randall1 and StenseL1 for part of the story.

The simplest process flow sheet for biological phosphorus removal incorporates two bioreactors in series, with the first being anaerobic and the second aerobic, as shown in Figure 7.38. This process was first presented in the open literature by Barnard who termed it the Phoredox process to indicate that phosphorus removal will occur when a sufficiently low redox potential is achieved through use of an anaerobic zone. He reasoned that the anaerobic zone should be placed first in the process train to take advantage of the electrons available in the raw wastewater, just as is done in the MLE and Bardenpho processes of denitrification." This same configuration was patented by Air Products and Chemicals, Inc. of Allentown, PA under the trademark Anaerobic/Oxic,®* or A/'O process.'" The major difference between the Phoredox and A/O processes is that in the latter the anaerobic and aerobic zones are divided into a number of equally sized completely mixed compartments/' For use in this section, we will consider only a single compartment for each. However, a large number of process flow sheets are available for biological phosphorus removal, both alone and in concert with nitrogen removal. They arc discussed in Chapter 11.

The simple process flow sheet shown in Figure 7.38 provides an opportunity to observe some very interesting interactions among the various types of bacteria in wastewater treatment systems. In the MLE and Bardenpho processes we observed interactions between heterotrophs and autotrophs. Introduction of the anaerobic zone allows the specialized PAOs to interact with both of those groups. To introduce those interactions and the effects that they have on design of BPR systems, simulations were performed in ASIM* using ASM No. 2. As discussed in Section 6.14, ASM No. 2 is very complex because it seeks to incorporate a number of complicated processes that are not yet fully understood. Nevertheless, it is sufficiently conceptually accurate to illustrate several important points, and it is for that purpose that it is used herein. The limitations inherent in the assumptions of the model will be pointed out as needed as the simulated performance of a Phoredox system is discussed.

Figure 7.38 Schematic diagram of two CSTRs in series with all influent and all biomass recycle to the first reactor, which is anaerobic. Although not shown, solids wastage is from the second reactor, which is aerobic. The configuration simulates the Phoredox process.

Table 7.1 Wastewater Characteristics Used to Simulate the Performance of the Phoredox Process*

Component

Concentration

Inert particulate organic matter

Slowly biodegradable substrate

Readily (fermentable) biodegradable substrate

Volatile acids (acetate)

Oxygen

Soluble nitrate nitrogen

Soluble ammonia nitrogen

Soluble biodegradable organic nitrogen

Particulate biodegradable organic nitrogen

Soluble phosphate phosphorus

Soluble biodegradable organic phosphorus

Particulate biodegradable organic phosphorus

Alkalinity

25.0 mg/L as COD 125.0 mg/L as COD 30.0 mg/L as COD 20.0 mg/L as COD

0.0 mg/L as 0: 0.0 mg/L as N 16.0 mg/L as N 0.9 mg/L as N 5.0 mg/L as N 3.6 mg/L as P 0.3 mg/L as P 1.2 mg/L as P 5.0 mM/L

The system chosen to represent the Phoredox process, shown in Figure 7.38, contains two bioreactors in series with the first being anaerobic and the second aerobic. As done previously the system has a total volume of 250 m\ receives 1000 m Vd of wastewater flow, and has a biomass recycle rate of 500 m7d from the clarifier to the first bioreactor. In this case, however, 20% of the total system volume is allocated to the first bioreactor. The first bioreactor is assumed to receive no dissolved oxygen whereas the second receives sufficient oxygen to maintain the dissolved oxygen concentration at 2.0 mg/L. The characteristics of the wastewater entering the system are given in Table 7.1. The major difference between them and the characteristics used in the previous simulations is that the readily biodegradable substrate has been divided into two components, acetate and readily fermentable substrate. Acetate is found in many wastewaters, particularly if the sewers are septic, and plays a major role in the metabolism of the PAOs, as discussed above. The concentrations of the various constituents differ somewhat from the concentrations used in the previous simulations. They were chosen because they were listed in ASM No. 2" and ASlM.h With the exception of abiotic phosphate precipitation, which was "turned off" to demonstrate the effect of biological phosphorus removal alone, the default parameter values in ASIM* were used. They are very similar to those in ASM No. 2" but are not listed here because the entire model has not been presented, as discussed in Section 6.1.4. Interested readers should consult the original sources.

7.7.2 Effect of SRT on Steady-State Performance

The effects of SRT on the concentrations of various constituents in the second bioreactor (and the effluent) of the Phoredox system are shown by the solid curves in Figure 7.39. For comparison, the concentrations in a single CSTR with a volume of 250 m' receiving the same wastewater are shown as the dashed curves. Quantitative differences between those dashed curves and others presented earlier are because of

_""" Phosphate-P _ I \

~ I Phosphate ~ 1 Accumulating J 1 Biomass

1 i 1 1 I 1 1 1 1 1

i 1 i 1 ITT=T"T"~1

1 1 >\ ' 1 1 1 1 1 1 1

I.i 1 /■ \ l l "

~ / ^ Heterotrophic Biomass" .1.1,1.1,1

02468 10 02468 10 SRT, days SRT, days

0 0

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