A Biogas Reactor can efficiently treat different types of wastewater. It is an anaerobic treatment technology that produces a digested sludge (digestate) that can be used as a fertiliser and biogas that can be used for energy. Biogas is a mix of methane, carbon dioxide and other trace gases which can be converted to heat, electricity or light D.7 .Describes biological processes that occur in the presence of oxygen.
Describes biological processes that
occur in the absence of oxygen.
Common name for the mixture of gases released from the anaerobic digestion of organic material. Biogas comprises methane (50 to 75 %), carbon dioxide (25 to 50 %) and varying quantities of nitrogen, hydrogen sulphide, water vapour and other components, depending on the material being digested. Biogas can be collected and burned for fuel (like propane).Mixture of solids and liquids, containing mostly excreta and water, in combination with sand, grit, metals, trash and/or various chemical compounds. A distinction can be made between faecal sludge and wastewater sludge. Faecal sludge comes from on-site sanitation technologies, i.e. it has not been transported through a sewer. It can be raw or partially digested, a slurry or semisolid, and results from the collection and storage/treatment of excreta or blackwater, with or without greywater. Wastewater sludge (also referred to as sewage sludge) originates from sewer-based wastewater collection and (semi-)centralised treatment processes. The sludge composition will determine the type of treatment that is required and the end-use possibilities.Describes technologies for on-site collection, storage, and sometimes (pre-) treatment of the products generated at the user interface. The treatment provided by these technologies is often a function of storage and is usually passive (i.e. requires no energy input), except a few emerging technologies where additives are needed. Thus, products that are ‘treated’ by these technologies often require subsequent treatment before use and/or disposal. In the technology overview graphic, this functional group is subdivided into the two subgroups: “Collection/Storage” and “(Pre-)Treatment”. This allows a further classification for each of the listed technologies with regard to their function: collection and storage, (pre-) treatment only or both.Refers to the methods through which products are returned to the environment, either as useful resources or reduced-risk materials. Some products can also be cycled back into a system (e.g. by using treated greywater for flushing).A functional group is a grouping of technologies that have similar functions. The compendium proposes five different functional groups from which technologies can be chosen to build a sanitation system:
User interface (U), Collection and Storage/Treatment (S), Conveyance (C), (Semi-) Centralised Treatment (T), Use and/or Disposal (U).
A sanitation system is a multi-step process in which sanitation products such as human excreta and wastewater are managed from the point of generation to the point of use or ultimate disposal. It is a context-specific series of technologies and services for the management of these sanitation products, i.e. for their collection, containment, transport, treatment, transformation, use or disposal. A sanitation system comprises functional groups of technologies that can be selected according to context. By selecting technologies from each applicable functional group, considering the incoming and outgoing products, and the suitability of the technologies in a particular context, a logical, modular sanitation system can be designed. A sanitation system also includes the management and operation and maintenance (O & M) required to ensure that the system functions safely and sustainably. The solid and/or liquid material remaining after undergoing anaerobic digestion.
The utilisation of products derived from a sanitation system.
A colourless, odourless, flammable, gaseous hydrocarbon with the chemical formula CH4. Methane is present in natural gas and is the main component (50–75%) of biogas that is formed by the anaerobic decomposition of organic matter.
A sanitation system in which excreta and wastewater are collected and stored or treated on the plot where they are generated.
The means of safely collecting and hygienically disposing of excreta and liquid
wastes for the protection of public health and the preservation of the quality of public water bodies and, more generally, of the environment.

Waste matter that is transported through the sewer.
An open channel or closed pipe used to convey sewage. See C.3 and C.4
Used water from any combination of domestic, industrial, commercial or agricultural activities, surface runoff/stormwater, and any sewer inflow/infiltration.

A Biogas Reactor is an airtight chamber that facilitates anaerobic degradation of blackwater, sludge, and/or biodegradable waste. Treatment of wastewater takes place as it enters the digester. An active sludge layer within the digester biologically degrades inputs. Digested sludge is discharged from the overflow point at ground level. The digester chamber also collects biogas produced in the fermentation process. The digestate is rich in organics and nutrients, and is easier to dewater and manage.

Design Considerations

Biogas Reactors can be built as fixed dome or floating dome digesters. In the fixed dome, the volume of the reactor is constant. As gas is generated it exerts a pressure and displaces the slurry upward into an expansion chamber. When the gas is removed, slurry flows back into the reactor. The pressure can be used to transport the biogas through pipes. In a floating dome reactor, the dome rises and falls with production and withdrawal of gas. Alternatively, it can expand (like a balloon). The hydraulic retention time (HRT) in the reactor should be at least 15 days in hot climates and 25 days in temperate climates. For highly pathogenic inputs, a HRT of 60 days should be considered. Sizes can vary from 1,000 L for a single family up to 100,000 L for institutional or public toilet applications. Because digestate production is continuous, there must be provisions made for its storage use and/or transport away from the site.

Materials

A Biogas Reactor can be made out of bricks, cement, steel, sand, wire for structural strength (e.g. chicken wire), waterproof cement additive (for sealing), water pipes and fittings, a valve and a prefabricated gas outlet pipe. Prefabricated solutions include geo-bags, reinforced fibre plastic modules, and router moulded units and are available from specialist suppliers.

Applicability

Biogas Reactor technology is appropriate for treating household wastewater as well as wastewater from institutions such as hospitals and schools. It is not suitable for the acute response phase, as the biological environment needs time to establish itself. Biogas Reactors are especially applicable in rural areas where animal manure can be added and there is a need for the digestate as fertiliser and gas for cooking. Biogas Reactors can also be used for stabilising sludge from Pit Latrines S.3 S.4 . Often, a Biogas Reactor is used as an alternative to a Septic Tank S.13 , since it offers a similar level of treatment, but with the added benefit of producing biogas. However, significant gas production cannot be achieved if blackwater is the only input or if the ambient air temperature is below 15 °C. Greywater should not be added as it substantially reduces the HRT. Biogas Reactors are less appropriate for colder climates as the rate of organic matter conversion into biogas becomes very low. Consequently, in colder climates the HRT needs to be longer and the design volume substantially increased. Even though Biogas Reactors are watertight, it is not recommended to construct them in areas with high groundwater tables or where there is frequent flooding.

Operation and Maintenance

To start the reactor, it should be inoculated with anaerobic bacteria (e.g. by adding cow dung or Septic Tank sludge). Digestate needs to be removed from the overflow frequently and will depend on the volume of the tank relative to the input of solids, the amount of indigestible solids, and the ambient temperature, as well as usage and system characteristics. Gas should be monitored and used regularly. Water traps should be checked regularly and valves and gas piping should be cleaned so that corrosion and leaks are prevented. Depending on the design and the inputs, the reactor should be emptied and cleaned every 5 to 10 years.

Health and Safety

The digestate is partially sanitised but still carries a risk of infection, therefore during digestate removal, workers should be equipped with proper personal protective equipment (PPE). Depending on its enduse, emptied liquid and sludge require further treatment prior to use in agriculture. Cleaning of the reactor can be a health-hazard and appropriate safety precautions (wearing proper PPE) should be taken. There are also dangers associated with the flammable gases but risks are the same as natural gas. There is no additional risk due to the origin of the gas.

Costs

This is a low to medium cost option, both in terms of capital and operational costs. However, additional costs related to the daily operations needed by the reactor need to be taken into consideration. Community installations tend to be more economically viable, as long as they are socially accepted. Costs for capacity development and training for operators and users must be budgeted for until the knowledge is well-established.

Social Considerations

Social acceptance might be a challenge for communities that are not familiar with using biogas or digestate. Social cohesion can be created through shared management and shared benefits (gas and fertiliser) from Biogas Reactors, however, there is also a risk that benefits are unevenly distributed among users which can lead to conflict.