Over 70% of municipal wastewater in Norway are treated with coagulation, with over 85% of the resulting sludge used in agriculture. The general targets of wastewater coagulation are maximum removal of particles and phosphates, but downstream processes may demand lower levels of phosphates removal and a lesser reduction of pH. Plant utilisation of phosphates in the sludge, which is a valuable ingredient in commercial fertilizer, is one of the motivations for high use of sludge in agriculture. However, research shows that plant accessibility of phosphates could reduce by over 80% after coagulation, in comparison to biological sludge. The strong chemical bonding between aluminium and phosphate ions is the suspected reason. On the other hand, ever increasing phosphate removal requirements push the plant owners to further increase the already excessively high Al:P dosing ratio, 3-7:1 moles. The result is - even less plant availability, unnecessarily high chemical costs, sludge management costs, and too low pH etc.<br/><br/>The common dosing control often bases on algorithms where the dosage is linearly proportional to the flow. They ignore the influence of particles, phosphates and pH as well as dilution factors due to precipitation and human behaviour. Two concepts that help manage these challenges use (1) improved algorithms to define the optimal dosage and (2) substitute inorganic (Al/Fe) coagulants with cationic polymers for colloids removal, alongside promoting the phosphate precipitation reaction to hydrolysis process.<br/><br/>An innovative dosing control concept can document reduced dosage by over 30%. A combination of this process and substitution of inorganic coagulants with cationic polymer helped manage the increase of plant availability from 20% to 50%, although even higher yields were anticipated. The competition of the hydrolysis and phosphate precipitation reactions are identified as the limiting reason. The relative concentration of colloids/particles and pre-polymerisation ratio of coagulants are identified as influential parameters. Theoretical options to favour the phosphate precipitation process to hydrolysis will be discussed.

Competition between the hydrolysis-phosphate precipitation reactions in wastewater coagulation

256th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.