Aerobic and Anaerobic Reactor Configurations

Biochemical Environment  Aerobic Conditions: Oxygen is used as electron acceptor  Anoxic Conditions: Nitrate is the electron acceptor  Anaerobic Conditions: absence of oxygen and nitrate and organic compounds are converted to biomass, CO2, CH4 and H2S. Organic matter + H2O CH4 + CO2 + NH3 + H2S+ new cell

Why anaerobic Treatment  Process stability  Produced Methane can be used to produce energy  Produced amount of excess sludge is about 10 % of aerobic treatment. Hence, reduction of waste disposal cost  Low nutrient requirement (BOD/N/P is 100/5/1 for aerobic; 700/5/1 for anaerobic mo.s  No air supplementation, so lower operational cost  No off-gas air pollution  Biodegradation of aerobic non-biodegradable  Seasonal treatment is appropriate

Possible disadvantages of anaerobic Treatment  long startup  Alkalinity should be sufficient  Under mesophilic conditions, optimum temperature is 35 ºC  Nitrification not possible  Low kinetic rates at low temperature  If COD < 1000 mg/L anaerobic treatment is not practical economically  Effluent from anaerobic treatment is generally not acceptable for direct discharge and aerobic polishing step is needed.

Equilization basin wastewater Waste ? Aerobic Bioreactor Anaerobic

Anaerobic Reactor Configuratios Anaerobic Contact Stabilization: CSTR with cell recyle under anaerobic conditions.  Well-mixed reactor  suspended growth  Before settling tank a gas separator is used to make settling easier Advantages: Simple and inexpensive Disadvantages: System is not stable for shock loading and toxic compounds Settling is problem

Anaerobic Filters Attached growth systems In this type of reactor, waste enters in the bottom and flows through the rocks or plastic media used for biomass immobilization. Recirculation is used to dilute any toxic compound in the influent. The main limitations of reactor : accumulation of solids in the packing material (plugging). So, wastes containing high amount of suspended solids are not suitable for A.F channeling cost of packing material

Organic Loading Rate (kg/m3.d) HRT (h) Removal efficiency Table.1. Some Studies on the treatment of industrial wastewaters using A.F. Type of wastewater Tempr. (°C) Organic Loading Rate (kg/m3.d) HRT (h) Removal efficiency Sugar industry 35-37 - 12-36 55% Distillery wastes 35 15 72 90 Chemical process 37 12-15 22-30 80-90 leachate 0.2-0.7 30-40 d 90-96

Upflow Anaerobic Sludge Blanket Reactor (UASB)

This type of reactor was developed to avoid the main problems of the anaerobic filter. Flow is in upward direction. Biomass settles in the bottom usually in the forms of granule. Advantages: High biomass concentrations. Hence high organic loading rates can be applied. so excellent COD removals due to high biomass concentrations. Compared to Anaerobic filter, wastewaters with higher suspended solid concentrations can be applied. Disadvantages: Sludge granulation is complex and not fully understood process Biomass escape at the effluent at higher loading rates hybrit reactors are used to avoid this problem)

Fluidized and Expended Bed Reactors In these systems microorganisms grow on small inert particles such as fine sand or activated carbon High Recycle ratios are used to keep the particles in suspension The rate of liquid flow and the resulting degree of expansion of the bed (10-25%) determine whether the reactor is a fluidized or an expanded (less expansion) bed reactor limitation: high and uniform upflow so high pumping cost gas Effluent Recirculation pump ORP, pH probes Water-jacketed glass reactor carrier Glass beads wastewater

Table 2. Some treatability studies using UASB Type of wastewater Influent COD OLR (kg/m3/d) Temperature C HRT (h) % COD removal Efficiency Beer industry 1000-1500 4.5-7 20-24 5 75-80 Agricultural waste 11 000 2-5 30 48 70-65 Slaughter hause 2000-3500 4 19 85 Paper industry 1000 - 49 75 Sugar industry 4000-60000 20-25 28-32 92-95

Aerobic Treatment Systems  Activated sludge Process (CSTR with/wo cell recycle)  Contact Stabilization  Oxidation Ditch  Sequencing batch reactor (SBR)  Extended Aeration  Step feed

Contact stabilization wastewater mixing Contact Basin Secondary clarifier Sludge waste Stabilization Tank Air

SBR Fill  No settling tank, no sludge pumping  Aerobic/anoxic/anaerobic cycles for nutrient removal  process flexibility for bulking  Tolerant to shock loading  No washout React Settle sludge Draw Idle

Oxidation Ditch Nitrification and denitrification is also possible Typically operate in an extended aeration mode