A theoretical and practical evaluation of struvite control and recovery

Multifaceted benefits of magnesium hydroxide dosing in sewer systems: Impacts on downstream wastewater treatment processes

Magnesium hydroxide [Mg(OH)2] is a non-hazardous chemical widely applied in sewer systems for managing odour and corrosion. Despite its proven effectiveness in mitigating these issues, the impacts of dosing Mg(OH)2 in sewers on downstream wastewater treatment plants have not been comprehensively investigated. Through a one-year operation of laboratory-scale urban wastewater systems, including sewer reactors, sequencing batch reactors, and anaerobic sludge digesters, the findings indicated that Mg(OH)2 dosing in sewer systems had multifaceted benefits on downstream treatment processes. Compared to the control, the Mg(OH)2-dosed experimental system displayed elevated sewage pH (8.8±0.1vs 7.1±0.1), reduced sulfide concentration by 35.1%±4.9% (6.7±0.9mgSL-1), and lower methane concentration by 58.0%±4.9% (19.1±3.6mgCODL-1). Additionally, it increased alkalinity by 16.3%±2.2% (51.9±5.4mgCaCO3L-1), and volatile fatty acids concentration by 207.4%±22.2% (56.6±9.0mgCODL-1) in sewer effluent. While these changes offered limited advantages for downstream nitrogen removal in systems with sufficient alkalinity and carbon sources, significant improvements in ammonium oxidation rate and NOx reduction rate were observed in cases with limited alkalinity and carbon sources availability. Moreover, Mg(OH)2 dosing in upstream did not have any detrimental effects on anaerobic sludge digesters. Magnesium-phosphate precipitation led to a 31.7%±4.1% reduction in phosphate concertation in anaerobic digester sludge supernatant (56.1±10.4mgPL-1). The retention of magnesium in sludge increased settleability by 13.9%±1.6% and improved digested sludge dewaterability by 10.7%±5.3%. Consequently, the use of Mg(OH)2 dosing in sewers could potentially reduce downstream chemical demand and costs for carbon sources (e.g., acetate), pH adjustment and sludge dewatering.

Deciphering nitrogen removal mechanism through marine anammox bacteria treating nitrogen-laden saline wastewater under various phosphate doses: Microbial community shift and phosphate crystal

The effect of phosphate on marine anammox bacteria (MAB)-dominated anammox process in nitrogen-laden saline wastewater was first investigated. The activity of MAB was enhanced by dosing low concentrations of phosphate (5-30 mg/L PO₄^3--P), and the time of complete ammonium removal was shortened by 0.5 h. When PO₄^3--P exceeded 160 mg/L, the calcium magnesium phosphate precipitation was formed in the reactor. The contact between substrates and biomass was hindered by the sediments, and the nitrogen removal performance of MAB was also worsened. At 400 mg/L PO₄^3--P, the ammonium removal rate and nitrite removal rate decreased to 0.45 and 0.43 kg/(m³⋅d), respectively. During the 158-day operation, MAB was still the dominant strain, but its relative abundance decreased by 15.4% at 400 mg/L PO₄^3--P. Besides, the presence of sediments stimulated the production of extracellular polymeric substances and the maximum yield reached 11.25 mg/g⋅wet weight at 200 mg/L PO₄^3--P.

Struvite production from anaerobic digestate of piggery wastewater using ferronickel slag as a magnesium source

This study aimed to fully recover ammonia contained at a high concentration in anaerobic digestate of piggery wastewater (ADPW) by forming struvite. As magnesium and phosphorus sources, ferronickel slag (FNS) and K₂HPO₄ were used, respectively. By leaching 200 g L^-1 of FNS with 3.0 M H₂SO₄, 10,309 mg L^-1 of magnesium ions were extracted, and this acid-leachate of FNS (FNSL) also contained 5965 mg L^-1 of total iron. In order to simultaneously remove both high concentrations of organic matters in ADPW and iron in FNSL which were known to hinder struvite formation, the mixture of ADPW and FNSL was added with H₂O₂ at the H₂O₂/Fe molar ratio of 0.75 and pH 4.0. After Fenton reaction, removal efficiencies of COD and total iron reached 77.36% and 99.89%, respectively. Then COD and an iron-reduced mixture of ADPW and FNSL were added with K₂HPO₄ satisfying Mg:N:P molar ratio of 1.2:1:1.15 at pH 9.5 to produce struvite for 1 h. From 1 L of ADPW (2.21 g NH₃-N), 0.65 L of FNSL (4.65 g Mg²⁺), and 5.63 g of PO₄ ^3-P, 46.7 g of precipitates were obtained. Overall removal efficiencies of magnesium, NH₃-N, and phosphorus were 98.59%, 94.25%, and 99.97%, respectively. Obtained precipitates were analysed by using XRD, XRF, SEM-EDX and found to be struvite with impurities of potassium and metals. Additionally, the economic feasibility of FNS was assessed by estimating chemical costs of various magnesium sources.

Resource recovery assessment at a pulp mill wastewater treatment plant in Uruguay

Using a mathematical model, a resource recovery assessment was carried out at a pulp mill activated sludge wastewater treatment plant (WWTP) located in Uruguay. Through the evaluation of different scenarios, the potential production of methane from secondary sludge, with its inherent energy savings, and the recovery of phosphorus (P) as struvite were estimated. Considering the current WWTP configuration with a sludge retention time (SRT) of 32 days, and according to the model, which is a simplification of reality, the assessment indicates that the implementation of an anaerobic digester (AD) to treat the excess sludge can lead to a methane production of approximately 1736 m³ CH₄ d^-1, being a promising alternative to increase the WWTP treatment performance. Furthermore, the model predictions suggest that by shortening the SRT from 32 to 5 days, the methane production could increase by up to 5568 m³ CH₄ d^-1. If the methane produced is used to generate electrical energy to operate the WWTP, energy savings of about 88% can be achieved. Regarding the potential recovery of P as struvite, the addition of a struvite reactor could be an efficient option to recover approximately 1611 mg L^-1 of struvite (corresponding to a load of about 433 kg d^-1). By optimizing the process performance, these findings highlight the potential recovery of resources in pulp mill WWTP, while complying with stringent effluent discharge standards. In addition, further research activities such as pilot-test or detailed laboratory studies may be needed to validate the previous recommendations for industrial scale application.

Phosphorous in the environment: characteristics with distribution and effects, removal mechanisms, treatment technologies, and factors affecting recovery as minerals in natural and engineered systems

Phosphorus (P), an essential element for living cells, is present in different soluble and adsorbed chemical forms found in soil, sediment, and water. Most species are generally immobile and easily adsorbed onto soil particles. However, P is a major concern owing to its serious environmental effects (e.g., eutrophication, scale formation) when found in excess in natural or engineered environments. Commercial chemicals, fertilizers, sewage effluent, animal manure, and agricultural waste are the major sources of P pollution. But there is limited P resources worldwide. Therefore, the fate, effects, and transport of P in association with its removal, treatment, and recycling in natural and engineered systems are important. P removal and recycling technologies utilize different types of physical, biological, and chemical processes. Moreover, P minerals (struvite, vivianite, etc.) can precipitate and form scales in drinking water and wastewater systems. Hence, P minerals (e.g., struvite, vivianite etc.) are problems when left uncontrolled and unmonitored although their recovery is beneficial (e.g., slow release fertilizers, sustainable P sources, soil enhancers). Sources like wastewater, human waste, waste nutrient solution, etc. can be used for P recycling. This review paper extensively summarizes the importance and distribution of P in different environmental compartments, the effects of P in natural and engineered systems, P removal mechanisms through treatment, and recycling technologies specially focusing on various types of phosphate mineral precipitation. In particular, the factors controlling mineral (e.g., struvite and vivianite) precipitation in natural and engineered systems are also discussed.

Total Value of Phosphorus Recovery

Phosphorus (P) is a critical, geographically concentrated, nonrenewable resource necessary to support global food production. In excess (e.g., due to runoff or wastewater discharges), P is also a primary cause of eutrophication. To reconcile the simultaneous shortage and overabundance of P, lost P flows must be recovered and reused, alongside improvements in P-use efficiency. While this motivation is increasingly being recognized, little P recovery is practiced today, as recovered P generally cannot compete with the relatively low cost of mined P. Therefore, P is often captured to prevent its release into the environment without beneficial recovery and reuse. However, additional incentives for P recovery emerge when accounting for the total value of P recovery. This article provides a comprehensive overview of the range of benefits of recovering P from waste streams, i.e., the total value of recovering P. This approach accounts for P products, as well as other assets that are associated with P and can be recovered in parallel, such as energy, nitrogen, metals and minerals, and water. Additionally, P recovery provides valuable services to society and the environment by protecting and improving environmental quality, enhancing efficiency of waste treatment facilities, and improving food security and social equity. The needs to make P recovery a reality are also discussed, including business models, bottlenecks, and policy and education strategies.

Struvite recovery from anaerobically digested dairy manure: A review of application potential and hindrances

Anaerobically digested dairy manure is rich in ammonium, orthophosphates, and magnesium, indicating a high potential for struvite recovery. Continuous generation of large amounts of dairy manure plus increasing global interest in anaerobic digestion of dairy manure suggest a huge market for struvite production with anaerobically digested dairy manure. However, the complex chemical composition of digested dairy manure presents hindrances to struvite recovery. This review paper assesses the significance and potential of struvite recovery from anaerobically digested dairy manure, identifies the factors hindering struvite recovery, and discusses the methods to overcome hindrances and the measures to improve phosphorus speciation of dairy manure for struvite formation. This paper proposes using "struvite recovery potential" or Pstruvite based on the least molar activity of struvite component ions in addition to "supersaturation ratio" to identify the potential for struvite recovery. The probable hindrances mainly include high Ca(2+) concentration and molar activity ratios of Ca(2+): Mg(2+) and Ca(2+): PO4(3-), high ionic strength, and high alkalinity. Struvite formation and purity is likely a function of all the interfering variables, rather than just a single factor with digested dairy manure. Potential enhancement measures need to be tested for technical and economic feasibility and applicability to various sources of digested dairy manure. This review paper provides guidance to overcoming the hindrances of digested dairy manure to struvite formation.