ABSTRACT
The major contaminants of wastewater that must be removed are biodegradable organic compounds, volatile organic compounds, recalcitrant xenobiotics, toxic metals, suspended solids, nutrients, microbial pathogens, and parasites. The domestic wastewater typically contains 800 mg of total solids /L, 240 mg of suspended solids /L, and 200 mg BOD /L. The BOD/N/P weight ratio required for biological treatment is 100/5/1. A waste sludge equivalent to 20–40% of the applied BOD is generated in the aerobic biological treatment of wastewater. The conventional activated sludge system includes an aeration tank for the growth of aerobic microorganisms and biodegradation of pollutants, a settling tank for separation of biomass and effluent, and a selection tank of fast-settling microorganisms for the aeration tank. Mixed liquor suspended solids (MLSS) is the amount of organic and mineral suspended solids, including microorganisms, in the mixed liquor. MLSS in an aeration tank ranges typically from 2,500 to 3,500 mg/L. Mixed liquor volatile suspended solids are the organic portion of MLSS including non-microbial organic matter, dead and live cells, and cellular debris. MLVSS ranges typically from 65% to 75% of MLS. The food-to-microorganism (F/M) ratio is a measure of the organic load in the activated sludge system. It is expressed in kg of BOD per kg MLSS per day. Hydraulic retention time (HRT) is the average time spent by the influent liquid in the aeration tank. It is the reciprocal of the dilution rate, which is a ratio of the influent flow rate to the working volume of an aeration tank. Sludge age (or solids retention time) is the mean cell residence time of microorganisms in the system. Removal of nitrogen happens either through nitrification followed by denitrification or partial nitrification of ammonium to nitrite that is followed by the anammox process. Removal of phosphate happens either by chemical precipitation of Ca or Fe phosphates or by using an alternation of anaerobic and aerobic bioprocesses. The overgrowth of filamentous bacteria in activated sludge may be due to a low F/M ratio, a low concentration of dissolved oxygen, and a presence of H2S. Foaming in aerobic tank is due to (1) presence of undegraded surfactants; (2) excess of fat in wastewater; (3) excessive growth of actinomycetes producing biosurfactants. Foam control includes: (1) chlorination; (2) diminishing of sludge age; (3) use of a biological selector; (4) reduction of aeration rate; (5) lowering pH, oil, and grease levels; (6) application of antifoam agent. Biofilm reactors for wastewater treatment include trickling filters, rotating biological contactors (RBCs), and submerged filters. The microbial biomass of biofilm is attached to the carrier. A tutorial with solutions and a quiz bank are added to this chapter.
