The Full-Scale Impact of Water Treatment Alum Sludge on Municipal Wastewater Treatment

The Robert O. Pickard Environmental Centre has a design capacity of 545 MLD and treats an average municipal wastewater flow of 430 MLD using conventional secondary wastewater treatment including chemical phosphorous removal. Primary and secondary sludges are treated through a combination of centrifuge thickening and dewatering, and mesophilic anaerobic digestion. In 2008, the plant started receiving up to 9 dry tonnes/day of alum sludge by sewer from the city's two water treatment plants (300 MLD combined flow). This new stream has had several significant impacts on the plant processes, operating strategies, maintenance and chemical costs. Sludge bulking in primary treatment has resulted in higher blankets to achieve comparable sludge density as prior to the change and longer solids retention has resulted in a greater tendency for septic sludge and odors. Also, approximately half of the aluminum is partitioning further downstream in secondary treatment to no apparent detriment to secondary clarification sludge volume indices. However, a sharp decrease in iron addition requirement for phosphorous removal took place along with decreased final effluent reactive phosphorous concentrations. At the same time, dewatering centrifuge performance was disrupted with a significant increase in polymer use to achieve acceptable though decreased cake dryness. This also resulted in increased cake volumes for disposal. Eventually process adjustments to centrifuge operation were made to optimize the use of polymer and cake dryness. Meanwhile, hydrogen sulphide concentrations increased in the biogas sent from anaerobic digestion to cogeneration. This paper details the cost and benefits of accepting alum sludge in a municipal wastewater treatment facility, its impact on the physical, chemical, and biological wastewater treatment process variables.