The process which we have named as the “Covered Anaerobic Lagoon Process” has been around since the development of geomembrane lining systems for low cost water storage. This method of encapsulating a lagoon to make use of the biogas has been in use since the 1950s, when plastic membranes first became available.
It was soon realised that for farmers and other site owners which create manure and also possess abundant land the use of these linings to form lagoons provides the lowest possible cost for large scale water storage in anaerobic digestion conditions. The rate of creation of biogas is low due to the lack of insulation and corresponding low temperatures, nevertheless, the installation cost of this type of biogas plant is also low.
Also, once the lagoon has been formed and lined, the provision of a floating geo-membrane cover to contain all odours is a simple matter. In most systems if allowed to inflate, the “floating cover”, will also provide a simple means of methane storage.
Much older and smaller versions of this process, which predate the advent of geomembranes have been around for several hundred years in China, and to a lesser extent India, where small earthen tanks provide sewage sludge digestion in a form which provides methane for cooking, and water heating.
The main modern day users of this type of covered anaerobic lagoon process plant have been industries producing high strength industrial wastewaters (eg the food processing industry), and the agricultural industry, to treat their sludges.
In the past the some covered anaerobic lagoon process systems were deployed primarily to provide water treatment, with additional aerobic post treatment prior to discharge.
However, the majority of covered anaerobic lagoon process systems have been installed to provide both a liquid effluent which can be sprayed on their land as a fertiliser and for their gas making ability, where the gas produced is usually be used within the facility as an energy source.
Currently lagoons of this type may be constructed more for their methane generation than in the past.
Typically for anaerobic lagoons the detention time might be anywhere in the range 20 to 50 days.
In cold climates the covers normally incorporate additional layers for heat insulation, and as the lagoons are generally not heated they tend to operate in the 15 to 25oC temperature range.
- relatively cheap and low technology
- all plant suitable maintenance by no-specialist fitters etc
- methane produced provide a very valuable energy source
- effluent has good fertilising quality and composted sludge has very good slow nutrient release characteristics.
- need constant attention to the quantity and mix of wastes in order to maintain a good quality biomass for treatment
- inaccessibility of mixers etc for maintenance in many designs
- gas could present an explosion or flashover risk and site should be risk assessed and an Explosion Protection Document produced for compliance with the ATEX (DSEAR in UK) Directive
- low temperature operation means long retention periods, so these plants are best suited to warm and hot climates.
The simplicity of these plants means that these can be built as a project using published design guidelines such as in Metcalf and Eddy, “Wastewater Engineering”. However, most would prefer to engage an expert and proprietary installers of these types of plant do exist.
Before installing a covered anaerobic lagoon process system check with your local environmental regulator to ensure that no permits or license is required to build one. If covers are added to slurry lagoons it is likely that in the UK, the SSAFO Regulations will apply.
Geo-membrane lined lagoons can possibly leak, and we recommend consultation with the local Environment Agency Officer, or a specialist consultant to ensure that you do not construct one of these lagoons in a location where groundwater quality is particularly susceptible to pollution if a leak did occur.
For example it would not be advisable to risk locating a slurry lagoon near a farm water supply well, or abstraction borehole.