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 mains water supply.
The choice of secondary treatment depends on several 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 is generally between aerobic or anaerobic treatment, although a combination of both methods can also be used.
In recent years we have seen a steady increase in the use of anaerobic digestion treatment techniques for the treatment of wastewater (and other effluent streams), but before we can examine what is driving this, 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 that use microorganisms to breakdown 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) when the treatment requires the presence of oxygen. 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 uses bacteria that 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 a large surface area for the treatment lagoon) and 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 several 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 biofertiliser
- AD plants generally have a smaller footprint than aerobic treatment
While the final choice of aerobic or anaerobic wastewater treatment will depend on the unique situation of each treatment facility, the advantages outlined above, together with greater utilization and uptake of AD technologies including enclosed digesters and upflow anaerobic sludge blanket (UASB) systems, means that the use of anaerobic digestion is rapidly increasing in the wastewater sector, either as the main secondary treatment, or to further process the biosolids produced by aerobic processes.
