Biological wastewater treatment methods are being used at an ever increasing extent for the treatment of wastewaters from the Finnish forest industry. Conventional activated sludge processes treating the composite wastewaters from the mill are most commonly used.

Present plants may turn out to be inadequate in some cases, or they may not serve the purpose. This can happen for example when 1) the process fails or the running costs become extremely high, 2) the legal regulations concerning the quality of the treated wastewater change or 3) production of the mill expands or is diversified. These situations can be handled by expanding the existing treatment capacity. An alternative is to develop specific additional treatment units more appropriate for the wastewaters in question.

In this paper the advantages and disadvantages of some biological process units completing activated sludge treatment plants are studied. In experimental studies a, b and c, a two-stage activated sludge process, a modified activated sludge process and an anaerobic pretreatment, respectively, are examined. The results of these studies show that a selective, highly loaded first stage of a two-stage activated sludge process can efficiently equalize and improve effluent quality, especially when the suspended solids concentration in the influent is high (Study a), that an anoxic/anaerobic unit together with a small aerated contact unit combined with a conventional activated sludge process can reduce phosphorus levels in the effluent (Study b), and that anaerobic pretreatment of concentrated wastewaters efficiently reduces the organic loading, improves sludge settling properties, and reduces sludge yield and phosphorus demand (Study c).

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