Introduction

Domestic wastewater constitutes a major component of wastewater generated everyday in developing nations like India, Bangladesh, etc. Untreated or partially treated wastewater finds its way to the water bodies resulting in enrichment of nutrients, leading to eutrophication. Algae based wastewater treatment plants or lagoons treat wastewater by natural oxidative processes. These treatment units consist of an anaerobic lagoon, facultative aerated lagoons followed by maturation ponds. Microbes aid in the removal of nutrients and are influenced by wind, sunlight and other factors.

Design of lagoons/pond systems have evolved with significant improvements during the last two decades. Investigations of hydraulic characteristics have helped in the modifications leading to more efficient ponds with aeration systems (Rinnhofer and Smith 2011; Olukanni and Ducoste 2011; Sah et al. 2012).  In this context, algal systems have revolutionized the pond concepts in imparting aeration by symbiotically living with bacteria with potential nutrient uptake rates. Several plant designs for biochemical oxygen demand (BOD) removal include facultative pond, complete-mix pond systems and anaerobic based lagoons to stabilize large quantum of organic solids in wastewaters (Abis and Mara 2003; Mara et al. 1992; Mara et al. 1996; Mara 2004). Computational fluid dynamics coupled with an optimization program and field based modeling (Olukanni and Ducoste 2011) have improved the pond design, configuration, flow paths that avoid short circuits at a minimal cost with optimal treatment efficiencies. Among other systems there are high-performance aerated pond systems, duckweed pond system, N removal pond systems and modified high-performance aerated lagoon systems for simultaneous nitrification and de-nitrification (Oswald 1990; Mara et al. 1996; Zimmo et al. 2003, Valero et al. 2010; Sah et al. 2012).

Waste stabilization ponds exploit symbiotic relationships between algae and bacteria in the removal of nutrients. In this regard, algae the primary producers generate O₂ (during photosynthesis) which aid in the efficient oxidation of organic matter with the help of the chemo-organotrophic bacteria. The type and diversity of the algae grown are potential indicators of treatment process (Amengual-Morro et al. 2012). Bacterial system disintegrates and degrades the organic matter providing the algae with an enriched supply of CO₂, minerals and nutrients. The stabilization pond based treatment system under investigation at Mysore depends largely on the algal-bacterial symbiosis for its treatment along with substantial odour reduction. The treatment plants are with the basic design comprising of facultative aerated lagoons followed by maturation ponds. During the investigation period, aerators were kept non-operational at Vidyaranyapuram treatment plant in Mysore to essentially assess the efficacy of algae based treatment options. Fermentative bacteria were being used for rapid conversion of organic matter to easily reducible substrates. The quantity of sewage generated in this catchment is about 150 MLD (million litres per day) and about 65% (95 MLD) is being treated, while the balance is tapped by farmers for irrigation. The wastewaters, before entering the treatment farm, are fed with fermentative-bacterial inoculums (OS-I & II).

The function of algae in ponds and its mechanism in enhancing the treatment efficiencies have seldom been addressed in sewage treatment systems analysis and most of the earlier studies (Jamwal et al. 2009) have focused on the efficiencies of treatment plant only.The objective of the present study is to investigate the algal dynamics in terms of abundance, distribution, cellular characteristics, bio-volume, carbon uptake efficiency and biomass densities for the possible uptake and assimilation of nutrients and analysis ofthe treatment efficiency of the Sewage Treatment Plant (STP) at Vidyaranyapuram.