Nowadays, waste disposal is a worldwide problem. In agricultural countries like India, waste discharges from agriculture, agrobased industries and city sewages are the main sources of water pollution. Conventional waste­water treatment systems do not seem to be the definitive solution to pollution and eutrophication problems. The major drawbacks are cost and lack of nutrient recycling (Eisenberg et al., 1981). Secondary sewage treatment plants are specifically designed to control the quantity of organic compounds in wastewaters. Other pollutants including nitrogen and phosphorus are only slightly affected by this type of treatment (Gates and Borchardt, 1964). Owing to the ability to use nitrogen and phosphorus for growth, algae can successfully be cultivated in such type of wastewaters (Mallick, 2002). This has been evolved from the early work of Oswald (Oswald et al., 1953) using microalgae in tertiary treatment of municipal wastewa­ters. The widely used microalgae cultures for nutrient removal are Chlorella (Gonzalez et al., 1997; Lee and Lee,

2001) , Scenedesmus (Martinez et al., 1999, 2000) and Spiru — lina (Olguin et al., 2003). Nutrient removal efficiency of Nannochloris sp. (Jimenez-Perez et al., 2004), B. brauinii (An et al., 2003), and Phormidium sp. (Dumas et al., 1998; Laliberte et al., 1997) has also been investigated. One of the well-known algae-based bioprocesses for wastewater treatment is high-rate algal ponds (Cromar et al., 1996; Deviller et al., 2004). Recently, corrugated raceways (Craggs et al., 1997; Olguin et al., 2003), triangular photobioreactors (Dumas et al., 1998), and tubular photobioreactors (Briassoulis et al., 2010; Molina et al.,

2000) have been developed for nutrient removal.

Among agroindustries, a large quantity of waste­water is generated from intensified aquaculture prac­tices. The main source of potentially polluting waste in fish culture is feed derived, mainly unconsumed and undigested feed and fish excreta. Discharging these
effluents directly into water resources causes eutrophi­cation of the receiving waters. Qian et al. (1996) reported the collapse of a prawn industry in China due to outbreak of pathogenic bacteria caused by high nutrient load. A few studies have shown the efficiency of algae biofilters in removing nitrogen from fish effluents (Cohen and Neori, 1991; Jimenez del Rio et al., 1996; Schuenhoff et al., 2003). These works are based on the use of seaweeds of the genera Ulva and Gracilaria to treat effluent water from aquaculture.

Recently, we intend to explore an integrated approach to produce biodiesel with simultaneous waste recycling by a green microalga S. obliquus with three types of wastes, viz. poultry litter (PL), fish pond discharge (FPD), and municipal secondary settling tank discharge. Our initial trial under laboratory batch culture condi­tions (Mandal and Mallick, 2011) encouraged us to conduct a small-scale field experiment in a recirculatory aquaculture system (RAS) using FPD and PL with the same microalga (Mandal and Mallick, 2012). Figure 11.1 presents a schematic diagram of RAS, developed at Agricultural and Food Engineering Department, Indian Institute of Technology Kharagpur, West Bengal, India. The effluent from a fish pond was pumped into a settling tank for removal of large solids. After 24 h, the superna­tant was siphoned to an inclined plate settler for removal of fine solids. To have a clear picture of the
inclined plate settler readers are requested to refer Sarkar et al. (2007). The effluent was then entered into fiber-reinforced plastic tanks (length 125 cm, breadth 60 cm, depth 45 cm) for culturing the test microalga.

FPD has a very high load of solid particles in suspen­sion, which contributes to increase in turbidity. Experi­ments carried on with sedimented and nonsedimented FPD showed that the nutrient removal efficiency of S. obliquus was higher in the sedimented one. Further ex­periments with sedimented FPD demonstrated that biomass and lipid yield was maximum at 15 cm culture depth with stirring. In seasonal variation study, the maximum algal biomass and lipid productivity was recorded during summer when sunshine hour was rela­tively large. During the summer season, when S. obliquus cultures pregrown in FPD supplemented with 5 g PL/l were transferred to the optimized conditions to maxi­mize the lipid accumulation (to have details on opti­mized condition readers are requested to refer Mandal and Mallick, 2009), lipid yield was raised by more than sixfold (up to 780 mg/l, Mandal and Mallick, 2012). Dur­ing rainy and winter seasons, comparable lipid yield was recorded by providing artificial lights for few hours. Thus an areal lipid productivity of 14,000 l/ha year (approximately) has been projected assuming 11 cultiva­tion cycles per year, leaving the rest of the period for cleaning and maintenance of the system