Covid-19 in sewage: Should effluent reuse be stopped?

Bengaluru has experienced unprecedented growth in recent decades. As population grew and infrastructure lagged, groundwater became over-exploited and untreated sewage was let into the waterways, resulting in severe pollution of its lakes and waterways.

Against this backdrop of water scarcity and inadequate wastewater treatment, the Karnataka State Pollution Control Board issued a Zero Liquid Discharge order in 2004. It mandates that all buildings with more than 50 residential units or a built-up area of more than 5,000 m2 in unsewered areas must install on-site Sewage Treatment Plants (STP) and reuse 100% of the treated water. Complying with this, many apartments have set up decentralised STPs and are utilising treated effluent for non-potable purposes such as landscaping, car-washing and flushing via dual piped systems.

Since the onset and spread of Covid-19, however, residents in high-rise apartments are increasingly sceptical of reusing treated effluent, for fear of contracting infection. Is this fear scientifically valid?

Some of this fear comes from media articles discussing the possibility of using sewage to quantify disease burden. The presence of Covid-19 RNA strands in raw sewage from areas with infection rates is well established. Scientists have shown that Covid-19 monitoring data from STPs can provide useful information about the onset of the infection before it is reported at the hospitals. This data can serve as an early warning system, help with tracing the location of the infected population, and to set up control/quarantine measures to curb the spread of the virus.

While Covid-19 has been widely reported in raw sewage, none of the studies have reported its presence in treated effluent. Wastewater treatment systems are by design meant to remove all kinds of harmful bacteria and viruses present in raw sewage. Even during pre-Covid times, raw sewage contained various pathogens that were eliminated by treatment.

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Broadly, wastewater treatment comprises of three steps: primary, secondary and tertiary treatment. Primary treatment is designed to remove large, suspended particles (organic and inorganic), followed by secondary treatment, where micro-organisms are employed to remove dissolved organic matter.

Tertiary treatment is deployed depending on the required end-use of the treated effluent. For example, if the treated effluent is to be used for landscaping, tertiary treatment involves disinfection only, with the objective to retain nutrients (N and P) that are crucial for plant growth, while eliminating harmful pathogens. If the treated effluent is to be discharged into a water body (pond/lake), it is crucial to deploy a tertiary treatment process to remove nutrients and pathogens. The objective here is to prevent eutrophication in the receiving water body.

Whatever the end-use is, disinfection is mandatory to eliminate pathogens and prevent the spread of infections. Chlorine – as bleaching powder, liquid chlorine gas, or sodium hypochlorite – is highly effective in killing pathogens present in treated effluent.

The WHO guidelines on wastewater management have not recommended any fresh measures as existing guidelines are enough to prevent Covid-19 infection via reuse of treated effluent. Ensuring wastewater treatment systems run at their design capacity and treated effluent is disinfected using chlorine is sufficient.

Although reuse of treated effluent is not considered a risk at present, there is some evidence of another possible infection pathway. Some studies have reported faecal aerosols as a potential pathway for the spread of Covid-19. In high-rise buildings, toilets, kitchen sinks, and washbasins are usually vertically aligned. The plumbing systems in such buildings act as a conduit for wastewater flow (down-flow) and aerosols (up-flow) within the apartment units. Piping systems connecting bathrooms (toilet and hand wash) are potential spaces for aerosol development. These are the spaces where the faeces become aerosolised.

During the SARS-CoV-1 outbreak in 2003, a study from China reported the spread of infection by aerosols developed in and transmitted via plumbing systems. Residents living in the apartment unit vertically aligned to the unit with infected people contracted the virus. However, this aerosol exposure pathway can be intercepted if: a) the infected person uses a separate bathroom, disinfects the bowl, shuts the bowl and ensures sufficient ventilation; b) the Resident Welfare Association is informed about which bathroom is being used by the infected person so that other residents can avoid using washrooms that are vertically aligned.

There is a concern that the treatment efficiency of an STP goes down if bleaching powder or any disinfectant is added to the plumbing system. In addition to killing pathogens, disinfectant in raw sewage also kills the heterotrophic bacteria that aids in removing organic matter. This issue can be fixed by maintaining higher concentrations of heterotrophic bacteria in the reactor, which can be achieved by lowering sludge wastage and maintaining high oxygen supplies in the STP.