Conventional wastewater treatment in many regions consists of three distinct phases: primary, secondary and tertiary. The primary treatment involves the mechanical removal of solids by sedimentation or flotation and is followed by a secondary treatment which removes organic matter through microbial decomposition. A further final, or tertiary, treatment may also be required depending on the final destination of the wastewater — such as re-entering the main water supply.
The choice of secondary treatment depends on a number of factors, including the wastewater’s chemical and biological oxygen demand (COD & BOD), operational and maintenance costs, sludge production, desired effluent quality, and microbial concentration.
The choice of secondary treatment is generally between aerobic or anaerobic treatment, although a combination of both methods can also be used. it is important to understand the differences between aerobic and anaerobic treatment, as well as the pros and cons of each.
Aerobic vs anaerobic treatment
Anaerobic and aerobic systems are both forms of biological treatment which use microorganisms to break down and remove organic materials from wastewater. The key difference between aerobic and anaerobic treatment is the presence of oxygen.
Aerobic treatment is typically applied to efficiently treat low strength wastewater (with relatively low BOD/COD values). In contrast, anaerobic treatment is typically applied to treat wastewater with higher organic loading.
In aerobic treatment, oxygen (air) is used to circulate the material, providing the right conditions for aerobic bacteria to reproduce. These bacteria assimilate and then break down organic matter and other pollutants, like nitrogen and phosphorus into carbon dioxide, water and biomass (sludge).
As the name suggests, anaerobic digestion utilizes bacteria which do not need oxygen. They break down organic material in the wastewater into methane, carbon dioxide and biomass (digestate).
Some of the factors in favor of aerobic treatment include the fact that it has less odors (as hydrogen sulfide and methane are not produced), and nutrient removal from the wastewater to the sludge can be more efficient — meaning that treated water can often be discharged directly into the environment.
However, oxygenation of the wastewater can require large amounts of energy or space. In addition, untreated biosolids can settle out from the process, requiring further treatment or disposal. The capital investment and space required for aerobic treatment is usually greater than that needed for anaerobic facilities.
While there are pros and cons to both approaches, anaerobic digestion (AD) has a number of advantages, including:
- AD is better at dealing with slurries with higher solids content
- AD produces biomethane gas which can be captured and used as a renewable energy source (including providing the energy to run the AD plant itself)
- AD produces less sludge (digestate) for a given volume of wastewater
- The stable digestate produced by AD is easily converted into a valuable biofertilizer
- AD plants generally have a smaller footprint than aerobic treatment
Ultimately, however, the final choice of aerobic or anaerobic wastewater treatment will depend on the unique situation of each treatment facility.
How heat exchangers improve anaerobic digestion efficiencies
As the points above show, one of the major benefits of anaerobic treatment is its improved energy efficiency and the lower volume of residual solids produced as digestate. However, when designing or upgrading an AD plant, there are numerous ways to maximize operational efficiency — improving both economic returns and environmental performance.
External digester heating using corrugated tube heat exchangers offers a number of advantages over heating systems which are located in the digester. External heating can be checked, cleaned or serviced at any time without the need to empty (or enter) the digester. External systems can be designed so that one heat exchanger array heats more than one digester, and the improved thermal performance reduces heating requirements and improves the overall energy efficiency of the AD plant. Operating life is often considerably greater compared to internal heating units, and routine maintenance is more straightforward.
Cooling and recovering the heat from exhaust gases can increase the efficiency of combined heat and power (CHP) plants used to generate electricity from biogas. Using heat exchangers on the exhaust recovers energy which can be used elsewhere in the plant, including feedstock and digester heating, pasteurization and digestate concentration.
A purpose-designed biogas dehumidification system can provide an efficient solution to cool and dehumidify biogas for combustion. The right system can condense up to 90% of the water contained in the gas, which is continuously separated before the clean biogas is ready for use as a fuel in the CHP engine, and an optional heat recovery step can reduce energy costs by up to 20%.
Care should be taken when choosing systems to pasteurize feedstocks of digestate. These systems are designed to effectively and efficiently pasteurize digestate, feedstocks, sludge, and similar materials, allowing operators to maximize the efficiency of their overall process while meeting regulatory requirements and increasing potential markets for digestate as a biofertilizer.
Traditional single tank pasteurization units simply dump this heat afterwards. It can be much better to use a system which recaptures this heat and uses it again, as they can be up to 70% more efficient than traditional single tank ‘heat jacket’ type pasteurization systems.
After digestion and biogas production, the digestate is often separated mechanically into solid and liquid phases.
A digestate concentration system based on evaporation offers a number of advantages at this stage. The volume is decreased, reducing the costs of storage, transport and application. Using a multi-stage evaporation process, the liquid digestate volume can be reduced by up to 80%.