Abstract
The water need of Chennai Metropolitan Water Supply and Sewerage Board (CMWSSB) to fully serve the said area by 2026 is 2248 million liters per day (MLD) whereas the maximum potential from surface water, ground water and sea water desalination (SWRO of 200 MLD) is only 1954 MLD. The cost of SWRO water is about Rs 55 per Kiloliter (Kl) whereas the tariff for public is just about Rs. 5 per Kl by Government subsidy which cannot be sustained for long. Out of 1810 MLD, 837 MLD is from Krishna River of contiguous state of Andhra Pradesh and 180 MLD is from Cauvery River of contiguous state of Karnataka. These are influenced by the vagaries of South-west monsoon of July to September and North-east monsoon of October to December. Thus, the only concordant reliable options are (a) more of SWRO by heavy financial burden or (b) indirect augmentation of aquifer by advanced treated sewage and is prudent. A pilot plant of 10 MLD has been commissioned at Chennai. This article cites this pilot plant and gives an overview for a fuller reuse of sewage by augmenting the riverine aquifer.
Demonstration of drinkability of renovated sewage by CMWSSB
Starting from 1965, it has taken a marathon half a century of fundamental and applied R&D to perfect the suitable treatment technology for the city sewage which has resulted in the powers of the higher echelons to voluntarily demonstrate their faith in the quality of advanced treated sewage being of drinking grade as in Fig.1.
Pilot plant for augmentation of public water supply source
This pilot plant is a 10 MLD plant. Its broad process flow sheet is in Fig.2 Though ultra filtration is supposed to take care of the smallest known organism namely the virus, still the ultra violet irradiation is provided in the downstream as an additional barrier to ensure organisms are not carried over. The activated carbon filtration (ACF) is provided in the downstream more as a traditional process unit for adsorption of traces of colour and its release requires thermal regeneration at about 700 to 900 °C in oxidative furnaces that decompose and oxidize the impurities and highly dependent on parameters such as temperature and duration. Other processes in various stages of trials are Pyrolysis and Microwave reactors which even if found to be successful do not fit into a water supply plant in public sector for the obvious challenges of tendering system for procurement, absence of Indian Standards to specify in the tender call etc in prototype application. This makes the situation as a “mountain of a molehill” as the residual organic colour can be easily bleached by the familiar Sodium Hypochlorite or chlorine in its Hydroxyl ions. Even the toughest reactive textile dye effluents have been debonded in its moiety. Similar is the case with Ozone gas which though has a light colour, does not have any odour and requires many safety precautions. All the same, these are “extra precautions” understandably incorporated when such a task is first undertaken. May be in prototype, these will give way to the simpler chlorine and not chlorine dioxide which is known to cause explosion in contact with ammonia. In the case of chlorine, the reaction with ammonia is mono, di and tri chloramines and which nitrogen is released into the ambience which itself has about 80 % of it. The bio monitoring as in fish survival in the product water and its discharge onto the lake is shown in Fig. 3. A moot point is the zero phosphorous as otherwise the 1300 ML water body may slowly go into eutrophication and will become unfit. This is manageable by adding Poly Aluminium Chloride (PAC) after the SBR and precipitate held in multi grade filter.
About Watson-Marlow Fluid Technology Solutions
The United Nations Development Programme (UNDP) way back in 1980, undertook the study of a river basin on the northern city and called the Kortalaiyar river originating at some 136 km on the west and draining eastward to the Bay of Bengal. Its f inal reach is skirting Chennai city at about 20 km. Except the North east monsoon months of October to December, the entire river basin is barren. This and subsequent studies have revealed a geological formation of a confined aquifer length as in Fig.4
The buried channel lithology was further studied for a limited 25 km from the sea as in Fig.5. At an average aquifer depth of 20 m and voids of 40 % in the sand, the total voids would be 10165 ML or about 6 days water storage or about 30 days for the full length. The entire renovated sewage flowing into a drinking water source as Porur lake can be let into this and close to 1600 MLD of water can be pumped back (Fig.6). The advantage is it is without evaporation loss after some initial two weeks of soil saturation and it will be a fill near the sea mouth and drawal along its length.
The economics relative to the seawater desalination
In contrast to Rs 5 per 1000 liters from treated sewage injection and extraction, the seawater desalination operational cost is Rs 55 per 1000 liters and is a dent in the balance sheet of CMWSSB as reviewed by Asian Development Bank in 2017 and extracted in Table 1.
Locations marked 1 to 4 are the sewage treatment centers placed around the city on outskirts. Already these are secondary treated plants, The centers marked as 2 and 4 already function with each 45 MLD operating plants with Tertiary Treatment and Reverse Osmosis (TTRO) beyond the secondary stage as in Fig.7. Thus a peripheral pumping main can collect the entire TTRO water and lead to injection point at sea mouth. TTRO sewage TDS will be 500 mg/l compared to 45,000 in seawater. Thus injected water will not seep out.
This 2017 report on the revenue vs. expenditure is based on the then water tariff at Rs 50 per month per dwelling. This has been increased subsequently to Rs 80 in 2019. Even then, in the foreseeable future in 2024, the gain is only 39 million USD which cannot offset the purchase of desalinated water at 62 million USD and which cost is at a 90 % subsidy to the public. It is a simple inference to recognize the futility of this forever, Moreover, other coastal towns and particularly the inland cities in the state which suffer the monsoon failures will raise their legal rights for similar subsided SWRO water. Thus it cannot go on forever.
Conclusions:
Given the physical and financial constraints and inter-state dependencies, Chennai Metropolis needs sewage TTRO for its full sewage and exploit the buried channel for a fill and draw reservoir as otherwise, the writing on the wall is clear that the population will emigrate and leave behind the huge present infrastructure as monuments. The model recorded here will also help similar water short coastal locations.
