Basic Decisions During Design And Evaluation

Design of a biochemical operation requires that decisions be made that are consistent with the guiding principles summarized in Section 9.1. Some of those decisions establish the nature of the facility, whereas others determine its size. Since the biochemical environment has a profound effect, its choice is one of the earliest decisions that must be made. Then the SRT is chosen, determining the various factors that follow from it, such as the mass of biomass, electron acceptor requirement, etc. Finally, the interrelationships between the bioreactor and the other unit operations in the sy stem must be considered.

9.3.1 Biochemical Environment

One of the fundamental decisions faced by a designer is whether to use an aerobic/ anoxic or an anaerobic operation. As discussed in Section 2.3.1, in aerobic/anoxic operations, heterotrophic bacteria use oxygen or nitrate-N as their terminal electron acceptor while using biodegradable organic matter as an energy and carbon source for growth. Furthermore, the presence of dissolved oxygen in such systems allows for the growth of autotrophic nitrifiers, which use ammonia-N as an electron donor, producing nitrate-N. In contrast, as discussed in Section 2.3.2. when both dissolved oxygen and nitrate-N are absent, alternative electron acceptors must be used. In fermentative systems, the biodegradable organic matter itself serves as the terminal electron acceptor, yielding soluble fermentation products, whereas in methanogenic systems carbon dioxide is the major acceptor, yielding methane.

Table 9.2 compares the features of aerobic/anoxic and anaerobic wastewater treatment systems. Both systems are capable of achieving high organic removal efficiencies. However, the effluent quality from an aerobic/anoxic system will generally be excellent while that from an anaerobic system will be moderate to poor. Aerobic and anoxic conditions allow extensive removal of biodegradable organic matter, particularly soluble material. In addition, the biomass in aerobic/anoxic systems is generally well flocculated, resulting in low effluent suspended solids concentrations. In contrast, although a high percentage of the biodegradable organic matter is converted

Table 9.2 Comparison of Aerobic/Anoxic and Anaerobic Systems

System

Feature

Aerobic/anoxic

Anaerobic

Organic removal efficiency Hffluent quality Sludge production Nutrient requirements Energy requirements Temperature sensitivity Methane production Nutrient removal

High High High

Low No

Possible

Excellent

High

High

Moderate to poor

Low to moderate

High Yes

Negligible

Low Low to methane and carbon dioxide in anaerobic systems, the resulting concentrations of soluble biodegradable organic matter can still be relatively high and the produced solids may be poorly flocculated. As a result, the quality of the effluent from an anaerobic system does not generally equal that from an aerobic system.

Waste solids production is high in aerobic/anoxic systems due to the large amount of energy made available for the synthesis of new biomass, resulting in relatively high yield values. Consequently, nutrient requirements are also high. In contrast, the biomass production and associated nutrient requirements for anaerobic systems are low because the relatively small amount of available energy makes the yield low. Power requirements for aerobic systems are high because oxygen must be transferred to serve as the electron acceptor, although this need will be reduced when anoxic zones are present. In contrast, the power requirements for anaerobic systems are low to moderate and generally represent the energy required to heat and mix the bioreactor. Heating requirements can be significant, but energy for heating is typically provided by the methane produced. Temperature control is critical in anaerobic systems because the methanogens are quite sensitive to changes in temperature. The performance of aerobic systems, on the other hand, is much less sensitive to changes in temperature. Finally, removal of nitrogen and phosphorus is possible in aerobic/ anoxic systems, whereas nutrient removal is negligible in anaerobic systems.

These features combine to provide advantages to aerobic systems for the treatment of low strength wastewaters and to anaerobic systems for the treatment of high strength wastewaters. Figure 9.2 presents the wastewater concentration ranges over which aerobic/anoxic and anaerobic bioreactors are typically applied and the ranges of HRT typically required. Both ranges are approximate and are provided only as general descriptors. The HRT range reflects both the range of SRTs required and the degree of separation between the SRT and the HRT achieved with each technology. Due to their ability to produce high quality effluents, aerobic/anoxic systems are typically used for wastewaters with biodegradable COD concentrations less than 1,000 mg/L. Although anaerobic systems can be applied to treat wastewaters in this concentration range, the effluent quality will generally not meet discharge standards, thereby requiring aerobic polishing. However, the combination of an anaerobic system followed by an aerobic system is usually not economical compared to a fully aerobic system for these wastewaters. In addition, low strength wastewaters typically result in insufficient methane production to heat the wastewater to the optimum temperature. Both aerobic/anoxic and anaerobic systems are used to treat wastewaters with biodegradable COD concentrations between 1,000 and 4,000 mg/L. Again, aerobic polishing of the anaerobic process effluent will be required if high quality is needed. Finally, in many instances the advantages of anaerobic systems outweigh the advantages of aerobic/anoxic systems for the treatment of wastewaters with biodegradable COD concentrations over 4,000 mg/L. The typical operating ranges for various anaerobic treatment systems are also presented in Figure 9.2. In general, low rate and high rate anaerobic systems employ biomass recycle to increase the SRT relative to the HRT, whereas anaerobic digestion systems do not, making the SRT and HRT identical.

Additional factors will also influence the relative economics of aerobic/anoxic versus anaerobic systems. Consequently, investigations must be conducted to distinguish the relative advantages and disadvantages of each biochemical environment for wastewaters with biodegradable COD concentrations near the overlap region in

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