This is the cover to a modern Klargester, a proprietary private foul drainage system. Commonly found in rural areas where it is not practical or too expensive to have a mains connection.
Otherwise known as a septic tank which works on the principle that discharged waste is collected and settled into a series of two or three underground chambers. Naturally occurring bacteria then helps to break down the material in the chambers. This is not usually fully effective due to the large volumes of discharge from modern households. Surplus liquid is dispersed into surrounding ground. An Environment Agency permit is required to do so.
Such systems are serviceable but can have limited success. Some smells and inconvenience may occur. Depending on the type of sub soil, the land containing the outfall drains can become saturated and soggy. The outfall drains can then become silted up and when this occurs they have to be taken up and replaced.
Whilst a correctly working septic tank does not need frequent attention, it will need periodic emptying by a competent drainage contractor to remove the accumulated material. The owner will be responsible for all maintenance costs. It is expensive to replace older systems, often running to at least several thousand pounds.