Integrated Wastewater Management for Tank Restoration in Urban Fringe Areas

Abstract

Urban fringe water bodies are often seriously affected by urban wastewater including partially treated sewage entering it and degrading it. Sustainability requirements necessitate that such wastewaters are treated and recycled locally while also the affected waterbodies are restored bringing back not only water quality but also a higher level of aquatic and aqua-faunal diversity. In semi-arid regions of peninsular India, man-made surface water bodies, also called as Kere or tanks are severely affected by the above phenomenon. There are over a 100,000 such water bodies and many are likely to face this danger. Therefore, new understanding, sustainable water treatment processes and eco-restoration options need to be evolved and this study attempts this need. As many cities in Southern India increasingly urbanize, small villages, towns and cities grow to ‘engulf’ nearby villages, the “Village Tanks” lose their importance as water sources accompanied by urban fringe wastewater flowing into them and resultant neglect, drying up or eutrophication. In the past most restoration efforts employed predominantly earth working to scoop out the silt /sludge. This situation prevails for decades until a complete sewerage and run-off networks are created. In this interregnum there is thus a need to restore waterbodies, which does not have any assured water source and thus needs treat locally flowing urban-fringe wastewater (UFWW) for which novel wastewater conditioning systems are to be developed. The proposed treatment /conditioning process needs to be preferably a passive one with little or no maintenance requirements. Therefore, to ensure adequate aquatic and lake-shore biodiversity, there is a need to tweak the UFW treatment systems to function passively, unattended for long periods and introduce adequate nutrition and primary components of the aquatic food chain and enable the food chain to establish in the water body. Towards this broader goal, a novel three-stage grid-independent, passive municipal wastewater conditioning system (PMWCS) comprising of an Upwelling Anaerobic Sludge Reactor for BOD removal followed by a micro-aerophilic Euglenoid Reactor for particulate (turbidity) removal and a third high-rate algal pond to reduce N and P levels and simultaneously disinfect water was conceptualized for urban fringe quality wastewater treatment. This study was inspired by findings of CST, IISc wherein 7 defunct STP of Mysuru, Karnataka, despite aeration failure, influent wastewater was getting treated in <3d HRT and this study attempts to emulate and optimize this finding. To evolve and characterize three zone (mentioned above), system was firstly tested and optimized at pilot scale (12L), then was scaled up to 1 million litres/d treatment capacity, to meet the needs of an urban tank - Puttenahalli "Lake", Yelahanka. Both pilot and field scale plants were operated and monitored for over one year to assess system’s feasibility and treatment performance. Finally, the functioning of the system at the waterbody level was monitored as “water quality” in the tank and characterized for the biological diversity, productivity and sustainability of the food chain now emerging in the restored waterbody (urban fringe tank, UFT). The objective was to achieve water effluent quality (as >mg/L) of COD 80; soluble BOD of 10, inorganic nitrogen 10, dissolved phosphorus 2 and anionic surfactant 1 by treating locally flowing UFWW in the abovementioned PMWCS. At the pilot scale, reactors were compartmentalized for the abovementioned three functions of BOD, turbidity, nutrient and surfactant removal and the underlying performance basis was characterized. This cascade was operated at variable OLR (0.5-0.75g/L.d), under various shock loads and starvation conditions. Results obtained show that all the design parameters were met with >90% BOD reduction, ~80% soluble N and P reduction and >80% SDBS reduction at HRT of 3-5d in pilot scale system and algal productivity of 4.69-5.75 g/m2.d. Algal productivity was an essential need as an input to the bottom of the food chain in the receiving tank while also enabling assimilation of the residual N and P. Additionally, the presence of higher forms of microalgae such as desmids in microaerophilic and micro algal reactors was a significant finding in surfactant fed reactors and highlights the potential of PMWCS in degradation of xenobiotics. PMWCS was scaled to 1MLD to suit restoration requirements for Puttenahalli tank, Yelahanka. A PMWCS comprising of a 3-reactor cascade as operated above was designed. It had a peak load capacity of 1MLD and functionality was understood in two phases. In phase 1 (12 months), the system was operated to compensate for the variable levels of evaporation losses across different seasons, thus maintaining a variable HRT of 3-10d. Along with this, stormwater intake and an intake weir were designed to ensure constant water level was maintained in the Tank. In the second stage (6 months), the system was run at a steady flow of 0.5-0.6 MLD with an HRT of 5-6d. At peak load (1MLD), the system was designed to potentially produce 80kg of algal biomass/day (in the HRAP) and another 120kg/d at the Tank Inlet zone (totalling 200kg/d) to kickstart the food chain in the lake. During field scale operation over a period of 18 months the PMWCS functioned satisfactorily achieving >85% BOD, turbidity, nutrient and surfactant removal. With changes in seasons and environmental parameters, under steady-state, no inhibition was observed at OLR of 50-350 g/m3.d and the sludge remained active with SMA of 0.7gCOD/gVSS.d. We observed significant treatment efficiency (>80 BOD5) at higher OLR which was evident with high sludge upwelling system and consistent with design feature. Using PMWCS + designated deep zone in the lake, ~85 and 80% of inorganic nitrogen and reactive phosphorus were also removed. Only ~1% BOD and 7% of input N&P went into the water body and thus prevented it from becoming eutrophied. Of the input nutrients, ~40% N and ~25% P was recovered as algal mass. On an average, the effluent was found to have 80-125 mg/L of algal biomass (with dominant Chlorophyceae and Bacillariophyceae). The residual nutrients and algal biomass become part of the aquatic food chain. The anionic surfactant, which cause havoc in tanks of Karnataka are successfully removed in PMWCS with effluent carrying <0.5 mg/L (90% reduction). The water quality in the lake responded well to the effluent from PMWCS and at steady state, despite the temporal and spatial variation BOD in the water was <10 mg/L, Hardness was <250 mg/L, dissolved reactive phosphorus was <1 mg/L and Inorganic nitrogen <5 mg/L. These results are quite favourable and better than all existing integrated wastewater management and lake restoration models. One of the main objectives of this work was rehabilitation of microbiota in the Puttenahalli tank. As expected, the residual nutrients and algal mass derived from PMWCS effluent initiated a stable food chain in the tank where 6 classes of organisms with genus richness of >80 were identified. These could be clustered into as 4 different trophic classes using Normalized biomass-size spectrum approach with high trophic transfer efficiency (~30%). Additionally, in treated UFWW fed system, prevalence of omnivorous species was observed. This adds to the overall stability of the food chain against any future anthropogenic or seasonal fluctuations. This restoration model is extendable to all urban fringe Tanks with no perennial water source or STP to fill up the Tank. This novel treatment system in addition to being used for UFT restoration could also be used for decentralized urban wastewater treatment.