Acid pretreatment is one of the critical pretreatments for the biorefinery. However, little information is available on water effluent that results from this acidic pretreatment compared to alkaline pretreatment. In this study, the wastewaters from an integrated modern magnesium bisulphite pulp mill (acidic) were utilized as models for the acid pretreatment effluents of a modern biorefinery. The coagulation technique, using high valency electrolytes, was applied for the treatment of treated and untreated process water from the pulp mill. The effectiveness of some electrolytes and the selectivity of the coagulation process were compared by measuring the removal extent of critical contaminants such as organics, phosphorus and colour. The salt coagulation process was found to be selective for colour and total phosphorus removal from the wastewater, achieving more than 90% removal of contaminants. (C) 2013 Elsevier Ltd. All rights reserved.