Urban Tanks for Facilitating Reuse of Municipal Sewage – A Case Study in Mysuru, Karnataka Academic research paper on "Agriculture, forestry, and fisheries"

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INTERNATIONAL CONFERENCE ON WATER RESOURCES, COASTAL AND OCEAN

ENGINEERING (ICWRCOE 2015)

Urban Tanks for Facilitating Reuse of Municipal Sewage - A Case

Study in Mysuru, Karnataka.

Vidya V. K.a*, Mysooru R.Yadupathi Puttya and Manjunath K. C.a

a Department of Civil Engineering, The National Institute of Engineering, Mysuru, India-570008

Abstract

Urbanisation is a very important factor leading to the deterioration of small tanks, which are a popular and decentralised means of runoff harvesting in the South Indian plateau. The study reported in this paper was taken up in order to develop a strategy that could effectively help conservation of urban tanks and facilitate also in utilise these precious water bodies of the previous generations as a means of recycling and reusing the domestic sewage. The strategy developed is illustrated considering Lingambudi Tank of Mysuru city, as an example. A small wetland is suggested as a pre-treatment option for reducing the pollutant load in the sewage let in to the tank. Area required to reduce the BOD5 content of the domestic sewage by 30% is estimated and has been found to be generally available in the vicinity of tank. A trial and error approach is adopted to determine the sewage inflow that can be managed, by considering the tank as a Stabilization pond. It has been found that the Lingambudi Tank can be easily maintained as a freshwater body catering to various needs, including recreation with water quality at bathing standards, with a sewage inflow of 5000 m3/day. Which turns out to be about60% of the annual freshwater inflow.

Keywords:Urban tanks; Runoff harvesting; Restoration; self-purification; GIS.

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Aquatic Procedia 4 (2015) 1508 - 1513

© 2015TheAuthors.Published by ElsevierB.V.This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of organizing committee of ICWRCOE 2015

1. Introduction

Small tanks, termed kere in Kannada, are a traditional means of harvesting runoff in the minor streamlets characterising the undulating topography of the plateau of South India.

* Corresponding author. Tel.: E-mail address: vidyavk2006@gmail.com

2214-241X © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of organizing committee of ICWRCOE 2015

doi:10.1016/j.aqpro.2015.02.195

These water bodies serve a variety of purposes, and the network of them that exists in the region is considered an unparalleled system of decentralised water resources development over the world (Palanisami, 2000; Dikshit, 1993). With the enormous growth of urban areas in recent decades, many rural tanks are also now in the midst of the urban conglomerates. As cities expand, tanks begin to face a multitude of threats, including inflow of sewage from new extensions, of industrial wastes and dumping of solid wastes. Also, the increased land-use conflicts in the urban setup result in converting tank beds into commercial complexes, housing colonies, exhibitions, bus-stands and other public utilities. Hence, urbanisation is a very important factor leading to the reduced benefits of the urban tanks. However, increased runoff, the hydrological effect of urbanisation, is beneficial to the tanks. Availability of consistent inflow of water in the form of sewage, of course in limited quantities, can also be considered beneficial to the tanks. But, when the quantity of sewage inflow is excessive the water pollution caused is devastating. Polluted tanks lose their purpose, are not only an eye-sore, but, eventually, form a health hazard too. Hence, it is necessary that great care is taken to see that urbanisation doesn't lead to deterioration of the tanks. But, although there is a plethora of policies and acts for the protection and restoration of urban tanks, tanks continue to slip into extremely poor conditions and succumb. Dumping of sewage in excess of quantities the tank can take continues to form a common practice, and hence has been the very practice of letting sewage in to the fresh water bodies is being dubbed hazardous. This probably because there are no proper strategies laid out for planning the quantity of sewage that may be safely disposed off in a tank and utilised beneficially. The present study has been taken up in order to develop a strategy that could effectively help prevention of loss of urban tanks and at the same time help recycling and reusing domestic sewage. The study is carried out considering the case of a tank in the city of Mysuru, which has yet maintained its tanks fairly well for storage, as an example.

2. Study Area

The city of Mysuru is famous throughout the world for its rich heritage, culture and tradition, which encompasses the tanks too. Presently, the city is spread over an area of about 195 km2, with a population of about one million, excluding the floating population. The normal annual rainfall of the city is about 780 mm. One of the important tanks within the city limits urbanised in the recent decades, the Lingambudi tank, has been considered for advancing the arguments and for illustrating strategy put forth in the present study. The location of the tank and the Google Earth view of the tank environs are shown in Fig. 1.

Fig. 1. (a) Location of the tank (Extracted from Toposheet); (b) Google Earth View of Tank.

The lake, streams and catchment boundary for Lingambudi tank are demarcated based on the information available in the Survey of India toposheets of the 1:25,000 scale. A supervised classification of the Geo-registered Google earth image of the year 2010 was carried out for obtaining the land-use land-cover (LULC) details. The tank is found to cover an area of 670084 m2, while the catchment area supplying runoff to the tank is 33.6km2.The stream network and the LULC details for the tank catchment are shown in the Fig. 2. The extents of the area in m2under the

various LULC classes areas follows: Water Body: 73499; Plantation: 1276633; Open Land: 7059694; Builtup Area: 3699988; Agricultural Area: 1685020; Eucalyptus: 3400786; Open Forest: 1026363.

Fig. 2. (a) Tank catchment and streams; (b) LULC details of the tank.

3. Studies on Tank Water Balance

Strategy to utilise best the tanks as a storage space for both the freshwater runoff and domestic sewage is to be developed on a knowledge of the quantities of these two inflows. The two inflows are estimated as below:

Runoff Simulation: The fresh water inflows are estimated by using the daily rainfall data of the typical year. The data for the typical year is generated stochastically, using a model developed by Putty and Hariprasad,1997.The popular SCS Curve Number method (Soil Conservation Service, 1972; Putty, 2010) is used to estimate runoff into the tank. As Lingambudi tank catchment is intercepted by the Bhogadi tank, the catchment area of the Bhogadi tank is excluded and the runoff calculations are carried out using the following CN values:Water Body: 100; Plantation: 85; Open land: 90; Bulitup Area: 91; Agricultural Area: 81; Eucalyptus: 89; Open Forest: 64.

Sewage Estimation: Sewage quantity is directly dependent on the amount of water used in the urban areas. It can be determined as equal to a part of the water supplied for domestic purposes. In the present study it is calculated as 80% of the per-capita water supplied, which is considered to be 180 liters per day in Mysuru. Considering the population density of the whole of the catchment as equal to that in an adjoining well developed locality, the sewage available is estimated to be 56000 m3/day.

Once the inflow details are estimated, a water balance study is carried out to understand the tank storage (S) details for each day. This is worked out by using the water balance equation:

Si = Si -1 + Inflowi-1 - (Evaporation + Percolation losses)i-1 - Outflowi-1 (1)

In which, the inflow may include runoff yield as well as sewage, and outflow is the quantity of water which is either withdrawn through the canals or the water which flows over the waste weir and the subscript i is the day number. Both evaporation and percolation losses depend on the surface area extent of the storage. Evaporation for any day is considered equal to the value normally adopted by the Water Resources Department of Karnataka in the region of Mysuru. The percolation losses are taken to be equal to 9 mm/ day, uniformly all over the year as is a practice in the department of Mines and Geology.

The dynamics of water balance considering runoff yield as the only inflow to the tank for the typical year is shown in Fig. 3. It is clear from the figure that with runoff as the only inflow, the tank will be at least half full for

less than four months in the typical year, even in the absence of withdrawals. Therefore, augmenting tank storage with sewage, to the extent of not going beyond the permissible limits of pollution, may be considered beneficial. Also, in order to make the best use of the tank, it is imperative that the inflows are utilised regularly. In this regard, it is necessary to plan a management strategy that permits optimum use of the tank, without the water body turning out to be a nuisance. The following is an attempt in this direction.

Fig. 3. Storage curve for the typical year with runoff as inflow.

4. Strategy Development

A strategy to conserve the tanks and harness sewage simultaneously should cater to the following requirements: (i) sustain a minimum storage of 30% of the tank capacity, (ii) utilise the maximum possible amount of sewage, which may, to be on the safer side, given a primary treatment upstream of the storage, and (iii) never permit deterioration of the tank water to the extent of permanent ecological damage. A number of parameters go to decide the amount of sewage that can be let into the tank in order to meet these requirements. They include (i) the capacity of the tank, (ii) the type of the pre-treatment proposed and the area available for the purpose, apart from the pollutants carried by the sewage, (iii) the purpose for which the withdrawal from the tank is used and (iv) the temporal distribution of rainfall and the freshwater inflows.

4.1 Wetlands for Preliminary Treatment

The wetlands, either constructed or natural, offer a cheaper and low-cost alternative technology for the treatment of wastewater.In general, a horizontal flow wastewater treatment systemshould have a 3-4: 1 length to width ratio and be rectangular in shape if minimal treatment area is available. A higher length-width ratio is required to ensure plug flow hydraulics (Miller and Black, 1985). The sewage flow rate is maintained by providing a V-notch. The required surface area of bed in m2 (A), for the Horizontal flow system wetland is calculated by using equation for the reduction of BOD5 in sewage effluent, proposed by Kickuth, 1977. It is expressed as,

A = F x Q x (ln Ci - ln Ce) (2)

where,Q - Daily average flowrate of sewage (m3/day);Ci - Daily BOD5 of the influent sewage (mg/l);

Ce - Required average BOD5 of the effluent sewage (mg/l) and F -is a dimensionless factor that depends on the depth of the bed and the biodegradability of sewage and is taken as 5.2 for readily degradable sewages.

The concentration of BOD5 for the domestic sewage generated in Mysuru is considered as 320 mg/l based on laboratory analysis at National Institute of Engineering, Mysuru and the wetland is designed such that it removes about 30% of the BOD5 from the sewage entering into the tank.

If the sewage is proposed to be let into the wetland by maintaining a uniform flow of 5000 m3/day the wetland area required is estimated to be 9273 m2, which is just about 1.4 % of the total tank area. Hence, a wetland can easily be established in the tank area and the so treated sewage let it into the tank for further purification. This

component of the water harnessed by the tank usually gets mixed with the largely unpolluted water present in the tank and gets self-purified in course of time, never deteriorating the tank beyond acceptable limits. However, it is required to be sure that even during the critical period, when the storage in the tank is low, the pretreated sewage entering in to the tank has sufficient opportunity to get treated, and hence that its quantities are not greater than what the tank can take. During such periods of low storages, the tank can be considered a stabilisation pond, for the sake of designing the permissible inflows. The design methodology suggested in the study is delved on below.

4.2 Tank Health as a stabilization pond

Waste Stabilisation Ponds (WSPs) are large, shallow basins in which raw sewage is treated entirely by natural processes involving both algae and bacteria. They are used for sewage treatment in temperate and tropical climates, and represent one of the most cost effective, reliable and easily operated methods for treating domestic and industrial wastewater. The following calculations are carried out in order to find the status of the tank taking up the sewage load considering tank as a stabilization pond.

As 30% of the BOD5 is removed in the wetland itself, the BOD5 of the sewage entering into the tank will be 224 mg/l (Co). Assuming that the effluent from the tank will have a BOD5 value of 20 mg/l (C) and oxygen production by algae for Indian conditions is 250 kg/Ha/day(Duncan Mara, 2013), the area and detention time required are estimated by the using following expressions:

Total organic loading on tank in kg/ day = (Flow X Influent BOD5) / 106 (3)

Area required in m2 = Total BOD loading /oxygen production by algae(4)

Detention time required = Kt/ Dispersion factor (5)

The value of Kt, the degradation coefficient in Eqn. 5, is obtained from Wehner Wilhelm curve (Metcalf and Eddy, 2003) using C/Co value and value of dispersion factor. For Indian conditions the dispersion factor is taken as 0.5.

On the other hand, the detention time available is estimated as Critical storage volume divided by the sewage flow rate. It is apparent that the quality of water in the tank is dependent on the sewage inflow as well the storage during the critical period. While the sewage inflow can be controlled, the latter value depends on the tank size, fresh water inflows, withdrawals and the sewage inflow itself. Hence, the permissible sewage inflows can be estimated only by a trial and error procedure. Such a procedure has been adopted in this study and the tank water quality has been investigated under two different scenarios, namely,

Scenario 1: Storage condition of the tank with rainwater and a calculated quantity of sewage diverted into tank; and no withdrawals are considered;

Scenario 2: Storage condition of the tank with rainwater and sewage as inflows and a calculated amount of water withdrawn continuously from the tank.

Fig. 4. shows the tank water balance under the two scenarios, during thetypical year, for a sewage inflow of 5000 m3/day and outflow of 2500 m3/day.Sewage inflow exceeding this quantity only improves storage of the tank but deteriorates the tank water quality.

Fig. 4. Storage curve for a typical year under two scenarios.

5. Results and Discussions

It is noted that the tank can be easily maintained as a freshwater body catering to various needs with a sewage inflow of 5000 m3/day into Lingambudi tank. The figure turns out to be 60% of the fresh water inflows. It is further found from Wehner Wilhelm curve (Metcalf and Eddy, 2003) that BOD5 value falls much below 4.5 mg/l.This is less than the desirable value 20 mg/l prescribed by Indian Standards for disposal of treated sewage into natural water bodies.

The yearly yield of runoff into tank for the typical year is 2285803 m3. This will not be adequate to maintain sufficient storage and tank will almost dry up during dry season. Hence, computations were made for a sewage inflow of 5000 m3/day with a wetland constructed for partial treatment of sewage. The analysis reveals that the tank can perform satisfactorily with this sewage loading. Furtherthe calculations have indicated that the tank can take a maximum sewage loading of 33005 m3/day. But this will require additional area for construction of wetlands near the tank premises.More over the tank characteristics will also change and tank will become a complete waste treatment plant. Since the main objective is to retain the tank characteristics without undue change in the quality of water,an inflow of 5000 m3/day of partially treated sewage is considered as safe and reasonable. This will also ensure a minimum storage of 30 % even during dry season in a typical year apart from providing 2500 m3/dayof water for irrigation and other beneficial uses.

6. Conclusion

The hydrological investigations of Lingambudi tank in Mysuru city indicate that the tank can be rejuvenated and maintained with adequate storage during dry season only with inflow of partially treated sewage. The strategy developed in this study can be applicable to other urban lakes in Mysuru City or elsewhere where the similar situation exists.Further studies on these lines could be suggested in order to develop readily usable guidelines to decide the quantities of sewage that may be harnessed in different cases.

Acknowledgements

This research work was carried out as a part of the a major R&D project - "Development of strategies for protecting and rejuvenating urban tanks - A case study in Mysuru", undertaken by The National Institute of Engineering, Mysuru, under the financial assistance of VTU.The authors acknowledge with thanks the funding by VTU. The authors also thank Miss. Samanvitha N., Miss. Lavanya G. and other N.I.E. students who have assisted the work.

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