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The activated sludge process introduces recycled sludge from the secondary clarifier downstream of the basin into the feed. The additional microorganisms and additional oxygen added to support them increases the efficiency of a basin beyond that of a simple aerated basin. Some activated sludge systems are enclosed and oxygen enriched air is introduced into the enclosure. The higher levels of oxygen, once again, increase the efficiency of BOD reduction. In evaluating the types of systems there is a trade off. As they become more efficient they become more complex and expensive to operate.