Wastewater sludge – a growing liability or existing resource? | Infrastructure news

Largely driven by legislation, wastewater sludge is increasingly viewed as a resource. Rudi Botha, senior water sector analyst at GreenCape*, talks to WASA about wastewater sludge beneficiation and its circular economy solutions.

“The Department of Forestry, Fisheries and the Environment, and the waste management divisions of municipalities largely consider waste as a resource. This perception has filtered into the water industry, where the energy and resource recovery from wastewater sludge is regarded as an attractive business model. However, our sector has not yet made the requisite shift where wastewater sludge transitions from a growing liability to a resource. Wastewater sludge is still a nuisance in terms of water management and is seldom a focus of the treatment process,” explains Botha.

“If sludge were viewed as a valuable resource and there were an emphasis on how to harness that resource, the treatment process would change. This would make it easier to recover the treated water from wastewater, as well as nutrients, energy and soil conditioners,” adds Botha.

Growing liability

 Transportation and disposal costs of wastewater sludge per metro by disposal method (GreenCape analysis, 2021)
The City of Tshwane has an agreement with a fertiliser manufacturer to process their sludge into compost for blending into fertiliser products. However, the other metros are facing transport and disposal costs of about R330 million per year. The disposal of wastewater sludges is an enormous cost to the operations of the wastewater treatment works (WWTWs).

The City of Cape Town (CCT), for example, currently spends around R60 million per year to dispose of (or apply to land) about 200 dry tonnes per day of dewatered primary and waste activated sludge (WAS) it generates, with an average moisture content of 83% (ranging between 58% and 92%).

This amounts to 74% of provincial sludge production. Half of the CCT’s sludge (the WAS) is in dry mass and applied to agricultural land, while the other half (primary and blended sludge) is treated and sent to Vissershoek private landfill.

Legislation – a driver to a more circular economy

Waste and wastewater discharge regulations, such as the ban on landfilling of liquid waste, and the Western Cape plan to divert organic waste from landfill (50% diversion targeted this year and 100% by 2027) are key drivers for resource recovery projects at WWTWs.

Furthermore, the 2017 amendment to Schedule 2 of the Electricity Regulation Act (No. 4 of 2006) provides the policy and regulatory framework for municipalities to develop their own electricity generation, such as biogas and combined heat and power (CHP) projects.

“A circular economy minimises waste; regenerates ecosystems; and keeps products, components, and materials, including biological materials, at their highest use and/or value for as long as possible. Municipalities, notably metropolitan areas, are large consumers of goods and services. As such, they are well placed to drive circularity at scale. Nowhere is this more relevant than the City of Cape  Town with its planned shift to wastewater sludge and digestate beneficiation,” highlights Botha.

Many WWTWs currently dispose of wastewater sludge at landfills. They will now have to find alternative, sustainable ways of sludge disposal. “In response to these two regulatory restrictions, the CCT is starting the transition towards anaerobically digesting its total wastewater treatment work sludges. Over the next 15 years, the CCT will be investing in the establishment of two regional biosolids beneficiation facilitieswith a third facility planned to serve future demand,” explains Botha.

Biosolids beneficiation facilities (BBFs)

As a result of the legislation, the CCT is transitioning towards anaerobically digesting its wastewater sludges in BBFs to:

  • produce A1a class treated digestate cake that is safe for unrestricted use, nutrient rich, odour free and low in contaminants
  • work towards sustainable sludge treatment, including electricity generation from biogas, reusable heat generation and recovery of nutrients
  • reduce its climate change liability.
A service contract (SCMB 82/11/20) is currently in place to dispose of (or apply to land) the CCT’s sludge. This contract will expire at the end of June 2023. However, it is expected that the CCT will re-tender for disposal, land application and/or beneficiation services for primary, WAS and blended sludge.

Once the first BBF has been commissioned, WAS and digestate cake will be available as part of the service contract, as well as emergency beneficiation/disposal of primary and blended sludge in case diverting is required.

All the primary and blended sludge will be digested at the first BBF, but there will be insufficient capacity to digest all the CCT’s WAS until the second BBF is commissioned (within 15 years). In the long term, an increased sludge production associated with population growth will require a third BBF facility or another circular solution for the WAS.

“What is challenging in the municipal space is sourcing funding needed to provide resource recovery facilities – BBFs have a large capital cost and there is minimal grant funding available for them. Due to the cost of the technology, large quantities of sludge need to be treated to allow for the efficiency of scale to be viable, for example, in metros,” explains Botha.

Sludge production rates per metro by disposal method (GreenCape Water MIR 2021)

The public perception and safety concernsregarding the use of wastewater sludge for energy production, agricultural application and saleable products to the public represent some of the major barriers to sludge beneficiation projects.

“As an example, it is acceptable to grow grass from beneficiated wastewater sludge, but there is still uncertainty around growing vegetables – especially regarding pharmaceuticals found in wastewater sludge,” says Botha.

She adds that there is also an issue around legislation. “While legislation has been a driver, it is also a barrier. There are a lot of private sector players that are ready to beneficiate organic waste, but legislation requires these companies to go through an environmental impact assessment process to get a waste licence. This is a huge cost and can take two to three years to obtain.”

However, the good news is that this process may be simplified for qualifying companies. Norms and standards for composting have been published and are being developed for organic waste treatment. Once promulgated, these norms and standards will streamline the whole process and will allow for qualifying facilities to operate without a waste licence, provided they comply with standard procedures and capacity limitations set forth.

Circular economy solutions

Options for beneficiating sludge and digestate identified in the vicinity of Cape Town
According to Botha, the composting of wastewater sludge is easier with smaller quantities, while stabilising the sludge and using it as part of fertilisers is being successfully done on a larger scale. “These fertilisers are registered and externally monitored. On the nutrient side of sludge beneficiation, one can utilise the carbon, nitrogen and phosphorus and put it back into soil to help plants grow. Various processes can be used to harness energy from sludges too.”

The increasing costs of electricity(27% over the next three years) will enhance the financial benefits of investing in sludge beneficiation to produce energy. This can be done via anaerobic digestion (biogas), pyrolysis, gasification, hydrothermal carbonisation, etc.

Some WWTWs have anaerobic digesters but they are often not heated nor run optimally, and the gas is not captured.

“While significant work has been done by the CCT to start shifting wastewater sludge from a liability to a resource, there has been exciting work in other provinces. Johannesburg has the Northern WWTW with anaerobic digesters and a biogas-to-energy facility (that was privately run) – the first in the country. However, the facility did not receive enough gas to produce a sustainable source of energy for the WWTW. Tshwane is investigating hydrothermal carbonisation. Both Johannesburg and Tshwane have successfully targeted the agriculture sector with their sludge. EThekwini has a number of anaerobic digesters, and some are fitted with biogas-to-heat facilities where the heat energy is reused within the digestion process. I am excited to see these developments and look forward to wastewater sludge being a bigger part of the circular economy,” concludes Botha.

*All GreenCape’s research into wastewater sludge beneficiation has been funded by the City of Cape Town.

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