Traditionally such refuse is either incinerated-requiring capital plant of high cost and causing some environmental pollution-or sent to landfill sites. The availability of landfill sites is decreasing alarmingly. Furthermore, the potential capacity of such sites is reduced by some 25% through the need to cap the fill with inert material on a daily basis to deny access to birds and prevent infestation. Since landfill sites are not contained within impermeable enclosures leachates and methane gas escape to cause pollution.

It is an object of the present invention to provide a waste management system which overcomes at least to some extent the problems aforesaid.

According to the present invention, there is provided a method for processing waste materials which include biodegradable matter comprising the steps of :- a) providing a plurality of storage cells for the waste materials; b) continually and sequentially filling each cell in turn under a cover; c) sealing each filled cell; d) collecting biogas from each cell as the biodegradable material therein decomposes; and e) emptying the earliest filled cell when N cells have been filled thereafter; the number of cells being sufficient having regard to the rate at which the waste materials are received that the residence time of the materials in any one cell is such that the material is completely or substantially decomposed when the cell is emptied.

There may be a total of (N+2) cells.

Collected biogas may be used to generate electrical and thermal power.

Some or all of the resulting thermal energy may be returned to the cells to accelerate the decomposition process.

Each cell may be of a capacity to receive a one year supply of waste from its intended catchment area.

The residence time of the waste in any one cell may be ten years.

The waste may comprise municipal refuse.

The waste may comprise a mixture of municipal refuse and sewage sludge.

The cells may be sealed by a sheet of rubber or plastics forming a roof-like cover.

The cells may be constructed from reinforced concrete having an impermeable base and impermeable side walls.

The cells may be constructed over a period of time as required as the initial cell filling operation progresses.

The emptied material may be disposed of by sorting, recycling including use as a compost or fertilizer material, reintroduction to a cell, transport to a landfill site or a combination of such measures.

Each cell may be filled in stages sections thereof being covered during filling and sealed after filling.

There may be two half-cell sized sections.

The invention will be further apparent from the following description with reference to the figures of the accompanying drawings, which show, by way of example only and in diagrammatic form, one plant for practising the method thereof.

Of the drawings:- Figure 1 shows a plan view of the plant; and Figures 2a-2e show the successive stages in filling one of the cells of the plant of Figure 1.

Referring firstly to Figure 1, it will be seen that the plant comprises twelve cells 10 (identified by the letters A-L). Each cell 10 is divided into half-cell sections 11 and 12 separated by an access roadway 13 for vehicular machinery.

The cells 10 are constructed from reinforced concrete having an impermeable base and impermeable side walls.

In this example, it is assumed that the plant will serve a community of 100,000 creating approximately 60,000 tonnes of municipal waste each year.

Each cell 10 has a capacity of 60,000 tonnes, typically having a length of 80 m, a width of 50 m and a height of 12 m.

Each cell 10 functions over a twelve year cycle. It is filled during the first year, the waste therein is digested during the following ten years and it is emptied and subjected to necessary maintenance during the twelfth year.

The cells 10 are continuously and sequentially filled in turn.

Referring now to Figure 2, an empty cell 10 is shown (Figure 2a). One of the half sections 11 is covered by an inflatable roof 14 (Figure 2b) and filled (Figure 2c) over a six month period. Beneath this temporary structure the working area would be weatherproof preventing the ingress of wind and water and the egress of dust, odours and waste. Infestation would be minimised When the section 11 is completely filled the roof 14 is transferred to the half section 12 and the half section 11 is sealed with an impermeable cover 15 designed to heighten anaerobic conditions and increase the generation of biogas (Figure 2d).

After the half section 12 is filled, the entire cell 10 is sealed (Figure 2e) with an expandable cover 16.

Biogas from sealed cell sections and cells is piped through lines 19 to an energy centre 20 and used to produce steam to drive steam turbine alternator sets for the generation of electrical power. Residual steam is returned through lines 21 to the cells to input thermal energy to maintain temperature of around 55-60° C to optimise anaerobic conditions and accelerate decomposition.

Material emptied from the cells 10 can be sorted, recycled (including for use as compost or fertilizer) reintroduced to a cell for further digestion, used as landfill on site or elsewhere or a combination of these measures.

It will be understood that construction of the plant can take place over a number of years from the commencement of the initial filling operation, cells being completed only as required for use.

It is of interest to note that 1 tonne of dry organic refuse can theoretically produce 780 m3 of methane and that 1 m3 of methane can theoretically generate 5kWh of electrical energy.

It will be appreciated that it is not intended to limit the invention to the above example only, many variations, such as might readily occur to one skilled in the art, being possible, without departing from the scope thereof as defined by the appended claims.

Thus, for example, the municipal waste can be mixed with sewage sludge or indeed selected categories of industrial waste.