Phosphorus recovery from anaerobically digested liquor of screenings

Phosphorus is a limited resource which is predicted to get exhausted at some point during the twenty-first century. However, it is present in wastewaters at concentrations that come close to supplying the nation’s annual requirements for fertiliser. Many papers have addressed the recovery of phosphorus as struvite (magnesium ammonium phosphate hexahydrate) from different types of waste while the most prominent usage of struvite is as a slow-release fertiliser, suitable as a replacement for chemical fertiliser, for agricultural application. In this study, screenings produced during the wastewater treatment process were anaerobically digested to obtain anaerobically digested liquor which was subsequently used for phosphorus recovery in the form of struvite. This was carried out at different concentrations of dry solids. The amount of struvite potential was calculated theoretically using molar ratio calculations of 1:1:1 (Mg:N:P). From the results, it was found that the digestate is high in phosphorus content and can be recovered up to 41%. For struvite yield, 0.27,kg of struvite can be recovered from each kg dry solids of screenings from 3% of dry solids. Screenings thus prove a valuable source of additional phosphorus which current disposal practices fail to exploit

Modelling of struvite chemistry

This paper shows the simulation results of struvite thermodynamics. The thermodynamic simulation shows the complex solution chemistry of struvite. Based on the thermodynamic simulation results, the complexes and ions present in struvite solution system are MgOH+, +, MgHPO4, MgPO4-, H3PO4, H2PO4-, HPO42-, NH3, NH4+, Mg2+ and PO43-. The thermodynamic simulation results also show that the supersaturation of struvite system is a function of reactant concentration (concentration of total magnesium, ammonium and phosphate) and solution pH. The paper also includes the validation the developed thermodynamic model using Minteq (a specialized thermodynamic modeling package)

Development and commissioning of controlled struvite crystallization in pilot scale

This paper focuses on the design of a nutrient recovery scheme, conducted in a fed-batch controlled crystallization technique, leading to the formation of struvite (Magnesium Ammonium Phosphate). The controlled crystallization technique included the control of solution pH and concentration during each experiment. Characterization of struvite crystal growth was conducted by CSD (crystal size distribution) analysis of growing struvite. Controlled crystallization showed significant growth of struvite crystals by surface diffusion, whereas faulty control initiated significant amount of fines

Struvite crystallization from nutrient rich wastewater

Discharge of untreated nutrient-rich wastewater is a problematic issue, which may\ud cause root burning and eutrophication of receiving water. It is also a problematic issue\ud due to the formation of crystalline deposits in waste water systems. The recovery of\ud nutrients using a crystallization technique may provide a value added product. The\ud recovered product is struvite, which is chemically known as magnesium ammonium\ud phosphate hexahydrate. The key focus of this research is the modeling and simulation\ud of struvite growth, which incorporates solution chemistry and thermodynamics, kinetics\ud of growth and process description of the recovery system. This research also focuses on\ud the strategy of struvite crystallization in a fed batch system, to avoid spontaneous\ud precipitation. A fully integrated control strategy in pilot scale is developed in this\ud research. This control strategy is based on feedback control, maintaining constant\ud supersaturation throughout the crystallization. The development and commissioning of\ud experiments includes investigation of suitable seeds, automatic temperature control,\ud operating zone of crystallization and correct design of the pilot scale reactor.\ud Experimental investigation showed a precise stability of the controlled supersaturation.\ud Moreover, size independent growth is indicated in this investigation. An ensemble of\ud experimental data is combined with a dynamic model to carry out parameter estimation\ud of struvite growth kinetic parameters using gPROMS

Reusing treated effluent in concrete technology

In this paper, the feasibility of using treated effluent for concrete mixing was studied. Treated effluent from sewage treatment plants in Malaysia is currently being wasted through direct discharge into waterways. With proper water quality control, this treated effluent can also be considered as a potential water resource for specific applications. Two tests were carried out namely compressive strength test and setting time to determine the feasibility of using treated effluent for concrete mixing. The results were compared against the tests conducted on control specimens which used potable water. The results showed that treated effluent increases the compressive strength and setting time when compared with potable water

Nutrient recovery from piggery effluents

The present study aims to evaluate struvite precipitation potential, using the Visual MINTEQ modelling package. A solution speciation model of struvite precipitation in different phases has been developed. This model indicates that across a wide range of pH (6-14) various other crystals are formed along with struvite, whereas struvite formation is dominant in the pH range of 7.75-9.27. Solution speciation modelling indicates that a Magnesium Ion Selective Electrode can be used to infer struvite supersaturation, enabling better modelling and control of crystal growth kinetics. A brief thermodynamic investigation of the piggery wastewater collection pond will be performed using this model

A fed-batch design approach of struvite system in controlled supersaturation

This paper focuses on struvite (MgNH4PO4·6H2O) crystallization in controlled supersaturation. Struvite can be used as a slow-release fertilizer. Crystallization experiments were conducted using supersaturated solutions. The secondary focus of this paper is the design of a struvite recovery system in fed-batch-controlled supersaturation mode. The design and commissioning of fed-batch struvite crystallization included the determination of operating supersaturation of struvite crystallization, suitable seed materials and the composition of feed solution. Determination of operating supersaturation of struvite crystallization was conducted by two steps including thermodynamic simulation using gPROMS2 (process simulation software) along with a set of batch experiments. Investigation of suitable seed materials was also conducted using set of batch experiments. Two types of seed materials including quartz sand and struvite seeds were used in the investigation of seed materials. Composition of feed solution included the investigation of struvite solution chemistry using PHREEQC3 thermodynamic modeling package. Based on the previously investigated design approach, struvite crystallization in fed-batch system was conducted using a 44-L of reactor with 15-L of initial reactant volume

Crystallization of struvite from metastable region with different types of seed crystal

The main feature of this paper was to recognize struvite crystallization in the metastable region of supersaturation. Thermodynamic equilibria of struvite were simulated to identify the minimum struvite solubility limit, thereafter validated by existing thermodynamic modelling packages such as PHREEQC and the derived data from existing struvite solubility curve. Using laser light scattering detection, spontaneous nucleation was identified by the slow increase of pH in a supersaturated solution of struvite. The crystallization experiment, conducted close to the saturation region in metastable zone, initiated struvite growth. The conducted experiment showed that mother crystal (struvite) was more effective as seeds for struvite crystallization

An approach of estimating struvite growth kinetic incorporating thermodynamic and solution chemistry, kinetic and process description

Discharge of untreated nutrient-rich wastewater is a problematic issue, which may cause root burning and the eutrophication of receiving water. It is also a problematic issue due to the formation of crystalline deposits in waste water systems. The recovery of nutrient using crystallization technique may provide a value added product. The recovered product is struvite, which is chemically known as magnesium ammonium phosphate hexahydrates. The key focus of this paper is the modeling and simulation of struvite growth, incorporating solution chemistry and thermodynamics, growth kinetic and process description of the recovery system. An ensemble of experimental data is combined with the dynamic model to estimate struvite growth kinetics