We consider inlet screens to be the front-line defence of a sewage treatment works, because by-passing rags can have an undesired impact on downstream equipment and the plant's overall efficiency. An inadequate or neglected grit plant can also lead to other inefficiencies - yet the water industry seems more tolerant - and in some cases allows for it. For example, a digester design may have up to 25% contingency capacity, just for the allowance of unwanted grit carried by imported sludge.
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