India faces a bleak scenario as far as sewage treatment is concerned. It generates thrice the amount of sewage compared to its treatment capacity. Even the capacity of treatment available on the ground is not being utilised to its fullest as the technology employed has already lived its useful life.
Mumbai, the commercial capital of India, fared no better than the country’s average. The sewage treatment plants (STPs) in the city operated on outdated, medieval technology, such as lagoon aerators, which consumed both land and power, and failed to meet effluent outlet parameters, resulting in polluting the sea and creating hazards during monsoons when the sea water comes back to inundate the low-lying areas of Mumbai and its suburbs.
The Brihanmumbai Municipal Corporation (BMC) released seven STP projects, which comply with all effluent quality norms, prioritise power efficiency and ensure environmental friendliness. BMC expects that these projects will finally bring the relief that residents have been looking for so long. Of these projects, JWIL has secured the prestigious Bhandup project and is involved in the development of an STP. The Bhandup STP, with a capacity of 215 million litres per day, represents a pivotal infrastructure project in Mumbai’s wastewater management system.
Adoption of CSBR biological treatment technology
The Bhandup STP project involves the implementation of cutting-edge continuous sequencing batch reactor (CSBR) technology, an innovative solution imported from South Korea. This marks a significant milestone as India adopts this technology for the first time. The CSBR technology was selected for its operational ease, low carbon footprint, lower energy consumption and overall low life cycle cost. It is a patented technology that amalgamates the A20 (anaerobic-anoxic-aerobic) process with a sequential batch reactor (SBR). It operates on a continuous flow basis, ensuring constant and modified flow within the SBR system. This approach allows for continuous operations while maintaining the benefits of the SBR process, such as versatility in handling varying influent conditions and ensuring high treatment efficiency. In addition, the A2O process ensures the biological nutrient removal (BNR) of phosphorous and nitrogen, and maintains a lower consumption of chemicals which would have been otherwise required to remove phosphorus.
In a CSBR system, wastewater undergoes treatment in sequential batches within a single reactor vessel. The process consists of several stages, such as filling, reaction, settling, decanting and idle phases. However, unlike traditional SBR systems, CSBR operates continuously, enabling a more consistent flow of treated effluent. The decanting system in CSBR works on the hydraulic (siphon) principle, which has several advantages such as zero head loss, zero interruption and maintenance-free actuator.
Working principle of CSBR
The continuous flow reactor (CFR) cells of the CSBR process operate the same way as the conventional or BNR tank-in-series activated sludge systems, with influent and mixed liquor flowing in and out of the cells continuously at a constant liquid level. All the equipment in the CFR cells run continuously without cyclic operation control. The two SBR cells, however, are operated alternately in either decanting or RBS mode on a cyclic basis, with all the equipment in the cells running automatically on and off in a cyclic manner. The CSBR process has continuous repeated operating cycles. Every operating cycle has two sub-cycles, each corresponding to the time required for an RBS-mode SBR cell to complete the process of return activated sludge (RAS)-based recycling, batch treatment and quiescent settling (or the time required for a decanting SBR cell to finish the process of decanting). If an SBR cell is operating in a sub-cycle RBS mode, it will become a decanting cell in the next sub-cycle and vice versa. The sub-cycle of the RBS cell is further divided into three phases – RAS recycling, batch treatment and quiescent settling.
Advantages of CSBR
CSBR technology provides two symmetrically configured SBR cells to alternately decant treated supernatant with continuous effluent discharge, eliminating the need for flow equalisation and minimising the size of downstream treatment facilities. It uses constant liquid-level operations, avoiding high head loss and low tank volume utilisation efficiency typical of variable-level SBR operations. Further, it provides dedicated reaction zones for efficient anoxic, anaerobic and aerobic treatment of wastewater with well-understood BNR mechanisms and a clear definition of each cell’s role. The technology also arranges these cells as non-major treatment cells at the effluent end of the system, minimising equipment duplication and significantly reducing system costs.
Class A sludge treatment
The project aims to achieve Class A sludge treatment. The Class A biosolids generated in the process will adhere to the US’s Environmental Protection Agency (EPA) CFR 540 guidelines. They offer numerous advantages in terms of environmental sustainability, soil fertility and cost-effectiveness, making them a valuable resource for agriculture, landscaping and other beneficial applications.
Reduced pathogens
Class A biosolids undergo treatment processes that effectively reduce pathogens to levels deemed safe for public health and environmental protection. This ensures that the biosolids can be safely used in various applications without posing a risk of disease transmission.
Versatile use
They can be used in various applications, including agricultural land application, landscaping and soil improvement projects. Their low pathogen levels and nutrient content make them suitable for enhancing soil fertility and structure.
Nutrient recycling
They contain valuable nutrients such as nitrogen, phosphorus and organic matter, which can improve soil quality and support plant growth. By recycling these nutrients, Class A biosolids promote sustainable agriculture and reduce the need for synthetic fertilisers.
Soil improvement
Their organic matter and micronutrients can improve soil structure, water retention and microbial activity. This leads to healthier soils, increased crop yields and enhanced environmental resilience.
Cost savings
Using Class A biosolids as a soil amendment or fertiliser substitute can result in cost savings for farmers, landscapers and municipalities. By recycling organic waste materials and reducing the reliance on chemical fertilisers, they offer a cost-effective solution for soil improvement and nutrient management.
Environmental benefit
They help to divert organic waste from landfills and incinerators, thereby reducing greenhouse gas emissions and conserving valuable landfill space. By promoting sustainable waste management practices, Class A biosolids contribute to environmental protection and resource conservation.
Regulatory compliance
Class A biosolids comply with stringent regulatory standards set by the EPA in CFR 40 Part 503. Meeting these standards ensures that biosolids are safely processed and managed, minimising potential risks to human health and the environment.
For the Bhandup project, thermal hydrolysis of sludge, followed by mesophilic digestion at 35-40 degrees Celsius, is being explored. This process involves pre-thickening the sludge to 10-14 per cent and then heating it under pressure at 160 degrees Celsius, ensuring a very high level of volatile solids destruction and eliminating pathogens. Once this cooked sludge is cooled down to 40 degrees Celsius and fed to mesophilic anaerobic digesters, a significant percentage of volatile solids are destroyed which, in turn, yields a much higher biogas production. The other process that is also being considered is high-rate thermophilic digestion, which involves anaerobic digestion at 55-60 degrees Celsius.
Biogas and cogeneration
Cogeneration from biogas is one of the most important pillars of this project. Biogas production from a digester is the result of the anaerobic digestion process, where organic materials such as animal manure, agricultural residues, food waste, or wastewater sludge are broken down by microorganisms in the absence of oxygen. This process produces biogas, primarily consisting of methane and carbon dioxide, along with trace amounts of other gases such as hydrogen sulphide and nitrogen. Biogas production from a digester offers a sustainable way to convert organic waste into renewable energy and valuable resources, contributing to environmental protection, energy security and resource conservation.
After scrubbing, the biogas is passed to biogas engines where gas undergoes combustion. Due to the enriched/conditioned methane content of a higher lower heating value, more power generation can take place. A biogas engine, equipped with a waste heat recovery system, a hot water generation system, a chiller and auto-control provision, is envisaged. Biogas engines are also designed to cater to emergency plant loads during grid power failure and can easily cater to emergency services of plants to achieve the required performance.
With lower power consumption by CSBR, extensive Class A sludge treatment leads to higher biogas production, and ultimately, more electricity generation. This will enable the Bhandup plant to move towards achieving a zero-carbon footprint.
