Paper mills are known as large water consumers. In many cases, the water in the process is being recycled many times in order to minimize fresh water consumption, steam requirement and effluent discharge. However, there are limits to the extent that the water can be effectively re-used. This can be due to technical limitations or to paper quality issues. One of the limitations that we have seen regularly in recycle mills is that when the effluent volume is minimized to the point that no or almost no effluent is treated, the dissolved COD in the process water increases and organic acids are produced causing odor in the final product as well as in the surrounding of the mill. Generally those mills prefer to maintain a certain amount of discharge and in these cases effluent treatment generally needs to be in place to comply to discharge standards. The reduction of effluent volume per ton of paper and therefore the fresh water consumption can go down further when the effluent is treated to such an extent that it can be re-used in the mill. This is beneficial from an environmental point of view because of less dependency from resources. With the integration of anaerobic technology in the treatment process methane is generated instead of using energy to convert COD via aeration. This results in a positive energy balance for the effluent plant. Additionally, if the dissolved waste is turned into green energy via methanization, it can boost the green/sustainability image of the company. Anaerobic treatment of recycled paper mill effluent is a proven and commonly accepted method. In this paper we will give some examples. In addition, a new generation of anaerobic bioreactors is presented, which is now being applied as the heart of effluent reuse, leading to more flexibility in load and increased gas production.
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