Improved external treatment of wastewater has become necessary for an increasing number of paper mills in view of stringent regulations governing the protection of receiving waters. Some of these advanced treatment processes have been shown to be useful after conventional primary treatment of wastewater with low concentrations of organics resulting from wood free paper production. For more contaminated wastewaters these state of the art techniques can also be used to supplement regular treatment like activated sludge processes. In these cases the advanced treatment processes are the final stage of a chain of mechanical, physico-chemical and biological treatment steps.

The equipment and processes used for advanced treatment are sand filters, biofilters, low-capacity trickling filters, flocculation and precipitation with inorganic salts in combination with filtration or flotation. These procedures are designed mainly for a reduction in concentrations of suspended solids, COD, and phosphorus.

Examples of the technical operation of these processes are presented. Compared to the results of standard treatment processes the results of the advanced treatment are a significant improvement As requirements for effluent quality will be maximized in industrialized countries in the near future - if this has not already been done - these processes will be needed, even if the costs are high.

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