Sometimes the ground is not suitable to allow effluent water to soak away. The soil may have become not be permeable if there is a lot of clay. Percolation tests need to be carried out to ensure that the water can be harmlessly disposed of. Tests should also be carried out following BS6297: 1983 to confirm the size of your soakaway. It must be sited 6 meters from a water course. Your soakaway should have a "Consent to Discharge" from the Environment Agency, who will advise you on how to perform the percolation test. Defective systems must be remedied without delay; if my memory is correct the fine can be up to �20,000 if your Environmental Officer decides to prosecute.