Re-thinking the treatment of sewage

More than ever, governments and municipalities around the world are taking the pledge of net-zero emissions with the end goal of limiting global temperature rise to 2 degrees Celsius above pre-industrial levels. Today, governments and nearly all utility companies are considering ways to reduce the amount of energy they purchase to treat sewage, and drastically reduce their greenhouse gas emissions (GHGe, measured as CO2 equivalents - CO2e). In the standard activated sludge process, the organics are broken down into carbon dioxide, methane and nitrous oxide through a biological process. Each of these gases can be broken down into global warming potential (GWP) equivalents:

• carbon dioxide GWP is 1 CO2e
• methane GWP is 21 CO2e
• nitrous oxide GWP is 310 CO2e

Aeration is a major player in the generation of these gases, and can consume up to 70% of the total energy used to treat sewage. Decoupling the aeration process has the potential of creating a net-zero energy requirement, however, it also goes against the standard activated sludge design principles.

Several decoupling process methods are being trialed by replacing the aeration process with various anaerobic processes; although functional, they are still very sensitive to incoming quality variations.

There is an alternative approach that is gaining interest within WSP’s treatment team, which is to move away from the conventional biological thinking of mainstream active sludge to a mechanical filtration approach. Within this approach, the screening of inert solids in still necessary, but most existing treatment plants already have inlet screens, degritters and even primary clarification, which are low energy users.

The filtration approach, following primary screening, would involve ceramic membranes, which have been around for 30 years and are now being considered for many different treatment options, as these offer over 20 years guaranteed life, are not fouled by oils and are averse to abrasion.

WSP’s basic design approach is in three stages:

The first stage is to decouple the biological process and filter the sewage using ceramic filter membranes to bring the feed concentration to around 6% solids content. This will feed the anaerobic digesters.

Early bench testing has shown 99% retention of solids, 80% retention of total chemical oxygen demand (COD) and 50% dissolved COD, where the particulate COD fractions are sent to the digester to enhance biogas production.

The second stage is the reduction of nitrogen, phosphate analytes and organics that cause colour from the liquid stream using ceramic nano-filtration, producing a final water quality suitable for reuse applications.

The third stage is how the process is made into net-zero neutral energy. This comes from the use of isobaric pressure recovery devices on the nano-filtration concentrate that transfer 98% of the pressure back to the feed flow. The use of potential or kinetic energy micro turbines could also be considered, since they would generate approximately 35 kW with a 72ML/d flow.

From the work WSP has undertaken globally, looking at ways to improve energy efficiency from peak shaving to waste stream energy recovery, it is clear that a sewage treatment facility using an activated sludge process treating 72 ML/d of sewage can require up to 1500 kW of energy to operate, but can only offset 250 kW through the production of methane (biogas), which is used to generate power.

Taking a simplified high-level view of energy generated vs. energy required, if the increased particulate COD is sent directly to an anaerobic digester, this would provide an increase in methane (biogas) production, which could generate an additional 100 kW. The estimated membrane approach would require approximately 380 kW. Therefore, the basic energy generated by the process is 350 kW (from biogas production 250 kW + the additional 100 kW) plus 35 kW (from a micro turbine), to give a potential 385 kW - i.e. net-zero energy.

There is a fourth stage, which considers increasing production for an energy surplus – meaning, turning the treatment of sewage into a power generator for the community. There are various options available that can increase the energy production from the addition of low temperature incineration of the final solid waste to generate heat. This allows the move from the standard Mesophilic digestion to the faster and higher methane-producing Thermophilic digestion. The heat can also be used to warm up the treatment buildings in the winter and provide cooling in the summer months. This can be coupled with selective combined waste digestion, using industrial/domestic organic waste, which is mixed to optimize the biogas production.
Taking a similar simplified high-level view of GHGe savings, i.e. the annual saving in GHG emissions from just decoupling the aeration process:

• CO2 generated from biological oxygen demand (BOD) oxidation = 1.1 kg CO2/kg O2 BOD oxidized. The reduction in GHG for a 72 ML/d plant would be approximately 7,200 tons per annum using a GWP of 1.0.
• Nitrous oxide has been calculated as 15.8 kg/d, therefore, the reduction in GHG for a 72 ML/d plant would be approximately 1,790 tons per annum using a GWP of 310.

Moreover, the expected reduction created through the removal of indirect emissions from the consumption of purchased electricity and gas will yield additional savings (calculated using 0.84 kg CO2e/kWh). Assuming the reduction is caused by the decoupling of the aeration process and the mixers and pumps associated with the activated sludge process, which would be 1500 kW total power requirement less the 385 kW for the membrane process, the reduction in energy requirement is 1115 kW for 24 hours a day, giving 26,760 kWh or 8,200 tons of CO2 per annum. The total GHG reduction would be approximately 17,190 tons of CO2e per annum.

A final advantage of the ceramic membrane approach is the space required. Comparing the physical footprint below of the ceramic membrane approach with only the activated sludge process (excluding any primary and secondary clarifiers), it is clear that the membrane approach can fit inside or above the existing activated sludge process.