Co-conditioning of the anaerobic digested sludge of a municipal wastewater treatment plant with alum sludge : benefit of phosphorus reduction in reject water

In this study, alum sludge was introduced into co-conditioning and dewatering with an anaerobic digested activated sludge to examine the role of the alum sludge in improving the dewaterbility of the mixed sludge and also in immobilizing phosphorus in the reject water. Experiments have demonstrated that the optimal mix ratio for the two sludges is 2:1 (anaerobic digested sludge: alum sludge; volume basis), and this can bring about 99% phosphorus reduction in the reject water through the adsorption of phosphorus by Al in the sludge. The phosphorus loading in wastewater treatment plants is itself derived from the recycling of reject water during the wastewater treatment process. Consequently, this co-conditioning and dewatering strategy can achieve a significant reduction in phosphorus loading in wastewater treatment plants. In addition, the use of the alum sludge can beneficially enhance the dewaterbility of the resultant mixed sludge by decreasing both the SRF and the CST, due to the alum sludge acting as a skeleton builder. Experiments have also demonstrated that the optimal polymer (Superfloc C2260) dose for the anaerobic digested sludge was 120 mg/l while the optimal dose for the mixed sludge (mix ratio 2:1) was 15 mg/l, highlighting a huge saving in polymer addition. Therefore, from the technical perspective, the co-conditioning and dewatering strategy can be viewed as a “win-win” situation. However, for its full-scale application, integrated cost-effective analysis of process capabilities, sludge transport, increased cake disposal, additional administration, polymer saving etc. should be factored in.