Introduction

Liquid waste generated in the domestic, commercial, industrial and agricultural sectors comprises of organics and inorganics constituents. Wastewater mostly consists of water (~99%) and solids (~1%). Numerous treatment processes are being adopted to treat wastewaters depending on the nature, type and extent of contamination. The treatment of wastewater is essential to minimise the contamination of land, water and soil. Treated wastewater can then be reused for various applications that helps in reducing freshwater usage. Treatment of wastewater requires an analysis of wastewater characteristics, which helps in finding appropriate technologies pertaining to the region. The quantum of domestic sewage generation has multiplied manifolds due to the rampant development, increased industrialisation and rapid urbanisation. It is estimated that water demand is about 42 x 106 MLD - million litres per day (National Commission for Integrated Water Resource Development, 2010) [1] (Figure 1) and about 33.6 x 106 MLD domestic wastewater is generated in India. The urban pockets are the potential generator of wastewaters i.e. 23,000 MLD and out of which 5000 MLD is being treated. This necessitates cost effective sustainable treatment options.

Fig 1 Water Supply(NCIWRD 1999) and waste water generatied in india

Urbanisation in India since last century shows an increase in urban population, the number of urban centres, and a rapid rise after 1951. The number of urban dwellers has risen to 285 million from 25.8 million persons in 1901, highlighting an eleven fold increase in urban population over the period 1901-2001 [2]. Due to increased population substantial volume of domestic wastewater is being disposed into surface water bodies resulting in water pollution and is a consequent threat to the aquatic biota and human health. Indiscriminate disposal of wastewater has contaminated the groundwater and soil adding to the existing miseries of humankind.
The population of India in expected to stabilise at ~1.7 billion people by 2050 [1]. As per the census of 2001 the urban population is 285 million, and keeping in view of the population projection for the year 2051, it is likely to be of the magnitude of 1093 million. Based on these, wastewater generation in 2051 would be about 132 billion litres per day [2]. Hence there is an urgent need for a proper, feasible, cost effective [3], less energy intensive wastewater treatment approach to suit the tropical climatic scenario while attaining maximum wastewater treatment in urban centres. However the associated costs need to be assessed before planning of the treatment plant projects [4, 5].
Large number of algae grow copiously in wastewater capitalising on available organic carbon and inorganic nutrients (N and P) and also play vital function of remediation by removing N and P [6-9]. The use of algae in wastewater treatment have been highly effective with the conventional oxidation (stabilization) ponds or the suspended algal pond systems such as high-rate algal ponds [10, 11, 12]. In algal systems, photosynthetic algae with O2 generation helps in aeration avoiding the requirement of energy and labour intensive mechanical aeration. Oxygenation of ponds through algae also aids in bioremediation of organic and inorganic compounds by heterotrophic aerobic bacteria [13]. Furthermore, algal based remediation does not generate additional pollutants and provides an opportunity for efficient recycling of nutrients while it is environmentally sound and sustainable option to manage wastewater.
Algae can grow at higher densities in highly concentrated wastewaters (Wang et al., 2010) to secondary treatment wastewaters often also used for tertiary polishing of wastewaters [13]. Algae transform wastewater C (organic/inorganic C) into algal biomass C. It has been reported that a substantial amount of this C is also found as lipids in certain wastewater algae [6, 8, 9, 14]. Lipid synthesis in wastewater algae provides additional benefits of algal biofuel development coupled with nutrient removal and wastewater remediation and ensures maximum resource utilisation [15]. Earlier growth studies by culturing algae in wastewaters, have revealed high biomass productivities with reasonable lipid yield [6, 8, 15]. Such investigations have also shown high potential of wastewater algae [6] and algal consortia [15] for removing C, N and P [13, 14]. This highlights the scope of wastewater grown algae for biofuel production and as potential alternate energy sources. Experimental studies involving wastewater-grown algae will help in addressing the driving factors to maximise biomass production, subsequent efficient harvesting for optimal lipid extraction. .
Algal biomass as a potential biofuel feedstock paves suitable path in exploring potential options to meet the energy shortfalls [6]. The dwindling stock of fossil fuels coupled with escalating oil prices and growing concerns towards greenhouse gases due to global warming and consequent changes in the climate has necessitated exploration for viable energy alternatives [16]. Challenges in algae based biofuel production are consistent supply of nutrients, harvesting of algae and effective lipid extraction techniques [17]. The fuel production involves a series of unit processes as algal species selection, mass cultivation, biomass harvesting, biomass concentration, lipid extraction and refining. This entails an understanding of algal downstream processing and process optimisation for its sustainable utilisation and commercial exploitation.
Studies on efficiency of the wastewater treatment plants based on water reuse and alternative source of water resources have used analytical benchmarking method of DEA - Data Envelopment Analysis [18]. Provisions are to be made to improvise the pond systems for better treatment efficiency and to improve the effluent quality and hence safeguard drinking water resources. Therefore detailed studies are required for the understanding the mechanism of the pond based systems to tackle the problem in best possible way which completely reduces the organics and nutrient load at the same time kills pathogens for its use in agriculture and other activities.
Earlier several studies have investigated the treatment plant efficiencies in many cities of India as New Delhi [19]; Indore [20]; Yamuna basin [21]; Bangalore west [22], Mysore [23] and in other countries as Algeria [24]; Greece [25]; Spain [26]; Iran [27]; Mexico [28]; South Africa [29]; Brazil [30]etc. These studies however do not address the socio-economic aspects of the treatment systems. Main focus of the present study is to assess lipid and bioremediation potential of algae, which involves:

• Evaluation of treatment efficiency of the large man made lake systems with facultative pond based systems and Mechanical STP (using extended aeration and/or activated sludge processes).
• Scope of biofuel production from algae growing in wastewater fed lakes and facultative ponds.
• Valuation of the wastewater system considering the capital, environmental and societal aspects.
• Formulating an alternative treatment option for urban cities.
• rank/grade the plant performance considering critical parameters based on efficiency criteria.