Phosphorous recovery through struvite precipitation from municipal wastewater

• Main Author: Lopez, Maristela Ann F.
• Format: text
• Language: English
• Published: Animo Repository 2021
• Subjects: Sewage—Purification—Precipitation; Sewage sludge precipitants—Recycling; Water—Phosphorus content; Chemical Engineering
• Online Access: https://animorepository.dlsu.edu.ph/etdm_chemeng/6; https://animorepository.dlsu.edu.ph/cgi/viewcontent.cgi?article=1005&context=etdm_chemeng
• Institution: De La Salle University

With the increase in stringency in the environmental limits for phosphates in treated effluent from wastewater treatment facilities, more phosphates are expected to be accumulated in wastewater treatment plants. These facilities are therefore opportune locations for recovering phosphorus bound in phosphates (PO4-P), as it is an essential nutrient in fertilizers. Struvite precipitation is then a simple nutrient recovery process which forms an insoluble compound with phosphates and ammonium dissolved in wastewaters. In this study, the operation of struvite precipitation in a wastewater treatment plant was modelled using process graph or P-graph methodology. The process network considered process alternatives for pH adjustment and seed material and was systematically optimized to minimize the net operating cost. The PO4-P recovery efficiency was then evaluated according to the amount of struvite produced. Untreated wastewaters were initially considered as the source for PO4-P recovery, and laboratory experiments were conducted to obtain data for the PO4-P recovery potential using these sources. Chemical hydrolysis pre-treatment using acid and alkali reagents were first performed to increase the solubilized PO4-P in the sewage and septage. The results showed that raw sludge contained mostly calcium phosphates, which are known to solubilize under acidic conditions. Modifications for the methodology of this pre-treatment were further needed, however, to be able to minimize the reactions forming insoluble compounds with the existing dissolved phosphates in the raw wastewater while the solubilization with the sludge solids is carried out, which was observed during alkali hydrolsis. Thus, an alternative source stream of sludge liquors was explored for struvite precipitation in the P-graph model instead. Three cases varying the PO4-P available for recovery in the treatment network were considered: treating sewage alone, co-treatment of sewage and septage, and treating septage alone. For an unseeded operation across all scenarios, the optimal solution networks used NaOH for the pH adjustment step in the precipitation process, while using sand as the seed material was shown to have the highest PO4-P recovery through the struvite produced, following either NaOH or aeration in the pH step.