Slow KWSB response, ageing sewer pipes main causes of urban floodingArchive
KARACHI: Heavy rains lashed Karachi on Aug 27, which resulted in flooded streets. Both the waste water (sewage) and rainwater overflowed their respective appurtenances (manholes and open drains). Why did this happen? Why it is so that such flooding of streets never occurs in Bangkok (except during 2011, when it was ‘wall of water’ everywhere)? The answer is simple, technically.
Karachi is experiencing a sanitation crisis that stems largely from poor governance. The Karachi Water and Sewerage Board (KWSB) tackles sewerage problems on an ad-hoc basis by addressing the immediate needs rather than long-term goals.
Less than 60 per cent of the population has access to public sewerage system, and almost all raw waste water finds its way to the sea, along with hazardous and industrial effluent.
How does the sewerage system work? In the ideal case, a sewer system is completely gravity-powered. Pipes from each house or building convey the waste water to a sewer main that normally runs along the side of the street. The sewer mains flow into progressively larger pipes until they reach the waste-water treatment plant. The waste-water treatment plant is usually located at the downstream end of the system. Manholes and ventilation pipes are provided at regular intervals for cleaning, and for release of built-up gases.
Most sewer pipes in the city have outlived their potential utility
Quite often, the topography of the land is not favourable to maintain gravity flow. In such cases, the sewer system will include booster pumps or lift stations to move the waste water from a lower level to high level.
Now, in that ideal case, if the sewer pipes are blocked, the waste water will start to build up, and eventually, will flow out of the manholes on to the streets. In simpler terms, this is what happens to Karachi’s sewerage system.
Topographically, Karachi has been clearly divided into two hydrological areas on account of the ridge running between M.A. Jinnah Road north and Drigh Road [Sharea Faisal] south of it. This ridge forms also a natural barrier between the Lyari and Malir rivers.
The causes of sewer blockages in Karachi are many. The old sewer pipes collapse, blocking the sewer line. Most of Karachi’s sewer pipes are old, and have outlived their potential utility.
Karachi has 250,000 manholes, most of which are uncovered. This allows debris to settle in the sewer pipes. During rains, the rainwater, which normally should flow in storm-water drains, makes its way into the sewer pipes, as storm-water drains are filled with solid waste.
At many places, the sewer pipes are unable to convey the waste water, as they don’t go the ultimate final point. The sewer pipe ends abruptly half way. This is the result of ad-hoc approach to the current sewerage problems. Technically, the sewerage and the storm-water drainage systems should be separated. But, this is not the case. This is one of the major reasons of flooding of streets in Karachi on Aug 27. Some people may intentionally create blockages, but this can be easily countered, if the KWSB has state-of-the-art vigilance system.
Sewer lines need periodic cleaning. This is because in many areas the water supply is deficient. This means that the waste water will be highly concentrated. The flow in the sewer pipe will not conform to the self-cleansing velocity, which is 2.25 feet/second, the minimum velocity of flow that will keep solids in suspension. If the velocity of flow in sewer pipes is less than the self-cleansing velocity, settling of solids will occur, which ultimately would lead to the blockage in sewer pipes. This is an important hydraulic point, but is never considered in the context of Karachi’s sewerage system.
About 60pc of Karachi’s population lives in katchi abadis (informal settlements; squatter settlements). They rely on onsite (on-plot) sanitation. Few katchi abadis have sewer system, but most have open drains. These are not connected to the formal sewerage system, as none exists. The waste water is disposed of in depressions and natural drainage areas. During heavy rains, the waste water, as a consequence, floods the streets.
Bangkok has superb systems for rainwater and waste-water flows. Bangkok maintains self-cleansing velocities in sewer pipes. Sewer pipes go all the way to the downstream end, without disjointing of pipes. Bangkok sewers are cleaned regularly. The waste-water treatment plants provide a high degree of treatment. Bangkok has seven super giant tunnels of five meters in diameter, which can convey rainwater at the rate of 13,435,200 cubic meter/day (2,955 mgd). When it rains in Bangkok, the water flows off the roads in minutes.
The KWSB has three waste-water treatment plants. These are located at SITE, Mehmoodabad and Mauripur with installed capacity of 232,000 cubic meters/day (51 million gallons per day); 209,000 m3/day (46 mgd) and; 245,000 m3/day (54 mgd).
The water utility had a fourth waste-water treatment plant, located in North Karachi. Designed and constructed by this writer in 1982, and based on the extended-aeration system, the plant, technically called aerated lagoons, with maximum hydraulic capacity of 22,730 m3/day (5 mgd), had the capacity to achieve satisfactory treatment efficiency.
In Karachi, an estimated 2,145,712 m3/day (472 mgd) quantity of waste water is currently generated. About 15pc of this volume is treated, and that also, to a poor degree of treatment. Here is why:
The treatment plants at SITE and Mehmoodabad were constructed around 1966. Designed by reputed foreign consultants, the plants were trickling filters. Trickling filters are high-class, reputed and robust plants, which produce high-treatment efficiencies.
In late 1980s, the KWSB degraded the two plants biologically, by removing the secondary treatment units. The degradation of plants, in terms of reduced treatment efficiency, is unheard of in the waste-water engineering field. The plants, after degradation, provided only primary treatment with around 20pc removal efficiency. The Mauripur plant will also not be able to treat modern-day emerging contaminants if the influent waste water contains toxic chemicals and industrial constituents. Karachi’s waste water contains industrial waste water.
Making functional the three plants, with their current technological stage, will not serve any purpose technically if the intended objectives are to control marine pollution, and control of water pollution in Lyari and Malir rivers, unless the waste-water treatment plants are upgraded and provided with secondary level degree of treatment. All the three plants need to be redesigned to provide secondary treatment with extended-aeration systems.
In order to fix Karachi’s sewerage system, and to prevent the flooding of streets with waste water during heavy rains, a number of measures have to be undertaken. The KWSB needs to be more active and, should be at the spot in no time.
According to an Asian Development Bank (ADB) report, it takes an average of 6.6 days for the utility to fix reported leaks.
The old and aging sewer pipes must be replaced on a priority basis and it should be ensured that the waste water flows all the way to the downstream end, and that the self-cleansing velocities are maintained throughout the system.
The first meeting of the provincial coordination and implementation committee (PCIC) held in Karachi on Sept 28 was informed that eight schemes of sewage treatment and disposal worth Rs162.6bn had been launched against which the provincial government from its own resources and the donor agencies had spent Rs12.42bn.
Further, it was told that the Greater Karachi Sewerage Plan, commonly known as S-III, was a Rs36.2bn project against which Rs10.70bn had been utilized so far.
Industrial waste water should be separated from the municipal waste water. Industries should have their own treatment plants. Polluter-pay principle should apply to industries. Sewer pipes, causing cross-contamination of drinking water, should be relocated.
One way to reduce the burden of waste water, and consequently its associated problems, is the adaptation of water conservation. The current waste-water generation of 472 mgd can easily be reduced to 1,363,800 m3/day, or 300 mgd, if the water metering is strictly enforced in Karachi.
Since katchi abadis contribute to the flooding of streets, the waste water generated by them should be brought within the control of sewerage system. This will also generate some revenue for KWSB. Poor sanitation spread pathogens, and other disease-causing organisms. Poor sanitation is responsible for 60pc of total diarrhoeal deaths.
Decentralisation of waste-water management systems should be adopted. This would, for example, mean that waste water generated, say, in scheme 33, Karachi University and Gulshan-i-Iqbal, is treated in that sewerage district. Treated waste water can be used for irrigation of parks, playgrounds, and green belts, in that sewerage district. Currently, the waste water of Scheme 33 is conveyed for a distance of nearly 25km, all the way to Mauripur. This is a worst-case scenario of waste-water engineering.
The Sustainable Development Goal (SDG) 6.3 says: by 2030, improve water quality by reducing pollution, eliminating dumping and minimising release of hazardous chemicals and materials, halving the proportion of untreated wastewater and substantially increasing recycling and safe reuse globally.
SDG 6.3 is vitally important, because poor sanitation in urban areas has a catastrophic impact on health of urban dwellers. It affects the whole city.
The writer has a master’s degree in water and waste-water engineering from the Asian Institute of Technology, Bangkok.
Published in Dawn, October 25th, 2020