KARACHI: The city currently generates an estimated 472 million gallons daily of waste water (sewage). The quantity of waste water treated is 50mgd and about 422mgd remains untreated.
Most people infected with Covid-19 will shed the virus, or pieces of the virus like RNA, in their stool. The virus can be shed in stool whether or not the person experiences symptoms. These pieces of genetic material can be detected in stool using molecular biology techniques.
While the detection of the virus material in waste water cannot identify specific individuals who are shedding the virus, it can identify that the virus is present in the community. The data are used to help understand the potential for community transmission among other uses, but is not a replacement for clinical surveillance.
This field of work is often referred to as environmental/waste-water surveillance or waste water-based epidemiology. Currently 45 countries and 182 universities, worldwide have programmes for waste-water surveillance.
KWSB needs to select sampling points throughout the sewerage system
The testing methods vary by laboratory (and country), but briefly involve collection of representative waste-water samples, concentration of the sample (by filtration or centrifugation), nucleic acid extraction, and finally a PCR-based detection method. There is no surrogate test for SARS-CoV-2 detection in waste water.
It is customary in waste-water engineering to report the finding of the analysis performed as number per millilitre (ml), as number of virus particles per ml. The greater the number, greater is the prevalence of virus in the community, and increase in clinical cases. If there is a decrease in clinical cases, waste-water data will show low virus particles per ml. Some countries found direct relationship of increase in viral particles per ml with increased hospital admissions. Most countries have developed appropriate models, which allow them to report findings as number per 100,000 inhabitants.
Karachi’s climate is more or less temperate. In temperate climates, it is hypothesised that SARS-CoV-2 transmission may increase during winter months, similar to patterns observed for the flu virus. Further, the virus is likely to be more stable and persist for longer periods at lower temperatures, which may also affect how long infectious virus survives in waste-water streams. Longer persistence during colder winter months may have little impact on waste-water surveillance.
Faecal shedding is highly variable from person to person and the patterns of shedding are not well characterized. Temperature may affect the number of people shedding SARS-CoV-2 and, therefore impact the amount of SARS-CoV-2 entering the waste-water stream.
Monitoring waste water for the presence of the genetic marker of SARS-CoV-2, its RNA, may be useful as a sensitive early indicator of low levels of infections in the community. Having an early warning system to alert public health officials about infection, or the lack of infection, could be helpful.
Waste-water monitoring for SARS-CoV-2 RNA has been shown effective to predict outbreaks of Covid-19. The virus recovered in waste water can sample whole city populations with results achievable in a reasonable time.
Presence of virus in waste water indicates that the virus is being shed by individuals in the community contributing to that waste-water system. Knowing the virus is present in waste water can be an important indicator when monitoring higher-risk communities. However, not detecting viral RNA in waste water can never be used to rule out the presence of infections in a community.
Furthermore, SARS-CoV-2 RNA concentrations in waste water cannot currently be used to determine the total number of infected persons in a community or the percent of the population that is infected.
Most countries have efficient sewerage systems and robust waste-water treatment plants. Those treatment plants, typically, have greater than 95 per cent treatment efficiencies. All the waste water that is generated goes to treatment plants for appropriate treatment. This makes their programme for waste water easy and straightforward. All they have to do is to take samples of waste-water treatment plants’ influent and effluent, and analyse those samples for viral particles.
Can KWSB perform waste-water monitoring?
This is not the case in Karachi. Out of 472mgd, only 50mgd is poorly treated. The waste water that goes to the Arabian Sea via Lyari and Malir Rivers is of the order of 422mgd.
The Karachi Water and Sewerage Board (KWSB) presently has three waste-water treatments plants, located at SITE, Mauripur and Mehmoodabad. It had a fourth treatment plant in North Karachi. Designed and constructed by this writer in 1982, the plant, technically called aerated lagoons, and based on the extended-aeration system, had the maximum hydraulic treatment capacity of five mgd (22,730 cubic meters/day). The plant achieved over 90pc treatment efficiency.
The KWSB needs to select sampling points throughout the sewerage system, ensuring that all areas — upstream, downstream, and in between — are covered.
For example, the Lyari River catchment areas start from Scheme 33 and North Karachi, and cover Gulshan-i-Iqbal, Nazimabad, Gulberg Town, and SITE. A sampling programme should be designed such that all these townships are covered. It will also serve the purpose of reporting cases in each township. This would be helpful for discussions and for presentations at appropriate forums.
There are two types of samples: grab samples consist of either a single discrete sample, or individual samples collected over a period of time not to exceed 15 minutes.
Composite samples are collected over time, either by continuous sampling or by mixing discrete samples. A composite sample represents the average waste-water characteristics during the compositing period. Various methods for compositing are available and are based on either time or flow proportioning.
In order to generate representative samples, the KWSB should go for composite samples. KWSB labs, especially the one at COD water treatment plant, may not have the analytical capability, in terms of equipment and trained staff. Large hospitals, on the other hand, do have these resources. Additionally, these hospitals may provide specific hotspots for waste-water sampling.
The writer has a master’s degree in environmental engineering from the Asian Institute of Technology, Bangkok.
Published in Dawn, January 20th, 2021