Understanding struvite crystallisation and recovery

Struvite crystallisation from wastewater effluents is seen as an alternative to traditional biological and chemical phosphorus removal processes used widely in the wastewater treatment industry. It presents the advantage of not only removing phosphorus but also generating a compound that could be reused as a fertiliser. However the application of struvite crystallisation processes at full scale is not widespread due to the unknown economical value of the process and the product, the need of pH control, the necessity of long operational times to ensure quality of the product and the formation of crystal fines. Preliminary crystallisation experiments were carried out at laboratory scale to provide a better understanding of nucleation and growth processes, and identify how basic parameters such as pH, mixing energy, water chemistry or presence of foreign ions affected struvite crystallisation. Particular attention was paid to the quality (i.e. size, shape and purity) of the crystal formed. The results revealed that the presence of calcium ions in solution could alter struvite purity and even inhibit its formation. pH was also identified as a parameter of major impact on struvite crystal quality. Indeed, pH could either influence struvite purity or affect size of crystals formed. Further investigations in a purposely built reactor also revealed that if struvite crystallisation is relatively simple to achieve, the control of struvite quality and more particularly crystal size is complex. Results at pilot scale showed that reactor operation and struvite surface charge could be a limitation to its agglomerative properties, hence to the formation of larger crystals. To optimise struvite crystallisation and limit the problem of fines formation the present study has investigated two possible solutions: struvite fines recovery by coagulation or struvite crystallisation on seed materials. Struvite coagulation proved to be an efficient solution to remove and recover struvite fines rapidly through floc formation. Of the coagulant tested, polyDADMAC was the most effective resulting in the formation of flocs 10 times bigger than the initial size of crystals. With regards to crystallisation on seed materials, the results revealed that success of struvite attachment onto seeds in short contact times was only efficient when mixing energy in the reactor was limited. In that sense, the submersion of a metallic system in the reactor allowed excellent phosphorus removal and rapid struvite recovery in only 2 hours of operation.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

What is the impact of personal care products selection on greywater characteristics and reuse?

Accounting for up to three quarters of the wastewater volume resulting from domestic activities but containing only a third of its organic content, greywater is seen as an alternative water source for non-potable reuse. This unique study explores the question whether consumers' product selection could affect the treatability and reuse of bathroom greywater. Fifty five personal care and household products (PCHP) were analysed for their effects on a range of water quality parameters including their aquatic and soil toxicity using Microtox® and MicroResp™. The organic content of these PCHPs varied considerably, not only from one category to another (0.2 gTOC.L−1 for hair conditioners to 2.7 gTOC.L−1 for toothpastes), but also within each category (0.1 gTOC.L−1 to 3.6 gTOC.L−1 amongst the shampoos). As expected, the PCHPs' macronutrient content was low, suggesting some limitation towards biological treatment of bathroom greywater. Regarding the impact of product selection on toxicity towards aquatic and soil microorganisms, the results revealed a higher sensitivity of Vibrio fischeri to the individual PCHPs than the MicroResp™ soil microorganisms. In the latter case, 75% of the products caused a stimulation response from the microorganisms although some decreases in basal respiration were observed for specific PCHPs within product categories. However, based solely on MicroResp™, the short-term discharge of treated bathroom greywater, regardless of consumer product selection, is unlikely to have a negative impact on soil microbial activity. Overall, the work has demonstrated the importance of consumer choice on the pollution load and treatability of greywater. However, no clear link between greywater characteristics and factors that normally determine consumer product selection (branding, type) were identified. This means it is not currently possible for consumers to actively manage the issue through choice such that process designers and technology developers must ensure technologies are sufficiently robust to manage the potential variations that could occur

Understanding struvite crystallisation and recovery

Struvite crystallisation from wastewater effluents is seen as an alternative to traditional biological and chemical phosphorus removal processes used widely in the wastewater treatment industry. It presents the advantage of not only removing phosphorus but also generating a compound that could be reused as a fertiliser. However the application of struvite crystallisation processes at full scale is not widespread due to the unknown economical value of the process and the product, the need of pH control, the necessity of long operational times to ensure quality of the product and the formation of crystal fines. Preliminary crystallisation experiments were carried out at laboratory scale to provide a better understanding of nucleation and growth processes, and identify how basic parameters such as pH, mixing energy, water chemistry or presence of foreign ions affected struvite crystallisation. Particular attention was paid to the quality (i.e. size, shape and purity) of the crystal formed. The results revealed that the presence of calcium ions in solution could alter struvite purity and even inhibit its formation. pH was also identified as a parameter of major impact on struvite crystal quality. Indeed, pH could either influence struvite purity or affect size of crystals formed. Further investigations in a purposely built reactor also revealed that if struvite crystallisation is relatively simple to achieve, the control of struvite quality and more particularly crystal size is complex. Results at pilot scale showed that reactor operation and struvite surface charge could be a limitation to its agglomerative properties, hence to the formation of larger crystals. To optimise struvite crystallisation and limit the problem of fines formation the present study has investigated two possible solutions: struvite fines recovery by coagulation or struvite crystallisation on seed materials. Struvite coagulation proved to be an efficient solution to remove and recover struvite fines rapidly through floc formation. Of the coagulant tested, polyDADMAC was the most effective resulting in the formation of flocs 10 times bigger than the initial size of crystals. With regards to crystallisation on seed materials, the results revealed that success of struvite attachment onto seeds in short contact times was only efficient when mixing energy in the reactor was limited. In that sense, the submersion of a metallic system in the reactor allowed excellent phosphorus removal and rapid struvite recovery in only 2 hours of operation

Kinetics of Struvite Precipitation: Effect of the Magnesium Dose on Induction Times and Precipitation Rates

The presence of white deposits in specific areas of wastewater treatment plants is generally the consequence of the spontaneous formation of a mineral called struvite. Struvite forms when the levels of phosphate, ammonium and magnesium naturally available in wastewater effluents reach a minimum molar ratio 1:1:1 under specific conditions of pH, temperature and mixing energy. Originally regarded as a phenomenon to be controlled or eliminated, struvite has been lately identified as an alternative way of removing and recovering P from wastewater effluents and generating a product identified as an excellent base for the production of slow release fertilisers. Chemical and physical principles of struvite precipitation and the development of crystallisation technologies have been widely investigated. However, little interest has been given to kinetics of struvite precipitation. In the present work the kinetics of struvite formation have been investigated at both laboratory and pilot scale in synthetic solutions containing Mg2+, NH41, and PO43- ions in a molar ratio 1:2:2 at room temperature. These different tests have used pH measurements to assess the impact of water chemistry on induction times, and more precisely the influence of magnesium levels on kinetic rates. Experimental results and kinetic calculatio revealed that the control of the magnesium dose initially present in solution is decisive of the speed at which struvite nucleates

Struvite crystallisation and recovery using a stainless steel structure as a seed material.

A metallic system acting as a seed substrate has been designed and developed in order to assess its efficiency in recovering phosphorus as struvite. The device, consisting of two concentric stainless steel meshes, was immerged in the upper section of a pilot crystallisation reactor fed with synthetic liquors (MgCl2·6H2O, NH4H2PO4,) for 2 h. Apart from soluble PO4-P removals which remained in the range 79–80% with or without application of the metallic system, it was found that under the specific operating conditions tested the meshes were capable of accumulating struvite at a rate of 7.6 gm−2 h−1, hence reducing significantly the amount of fine particles remaining in solution from 302.2 to 12 mg L−1 when compared to trials wit

Consumption-based approach for assessing the contribution of hospitals towards the load of pharmaceutical residues in municipal wastewater

Hospitals are considered as major sources of pharmaceutical residues discharged to municipal wastewater, but recent experimental studies showed that the contribution of hospitals to the loads of selected, quantifiable pharmaceuticals in sewage treatment plant (STP) influents was limited. However such conclusions are made based on the experimental analysis of pharmaceuticals in hospital wastewater which is hindered by a number of factors such as access to suitable sampling sites, difficulties in obtaining representative samples and availability of analytical methods. Therefore, this study explores a refined and extended consumption-based approach to predict the contribution of six selected Australian hospitals to the loads of 589 pharmaceuticals in municipal wastewater. In addition, the possibility that hospital-specific substances are present at levels that may pose a risk for human health was evaluated. For 63 to 84% of the pharmaceuticals investigated, the selected hospitals are not a major point source with individual contributions likely to be less than 15% which is in line with previous experimental studies. In contrast, between 10 and 20% of the pharmaceuticals consumed in the selected hospitals are exclusively used in these hospitals. For these hospital-specific substances, 57 distinct pharmaceuticals may cause concerns for human health as concentrations predicted in hospital effluents are less than 100-fold lower than effect thresholds. However, when concentrations were predicted in the influent of the corresponding STP, only 12 compounds (including the antineoplastic vincristine, the antibiotics tazobactam and piperacillin) remain in concentration close to effect thresholds, but further decrease is expected after removal in STP, dilution in the receiving stream and drinking water treatment. The results of this study suggest that risks of human exposure to the pharmaceuticals exclusively administered in the investigated hospitals are limited and decentralised wastewater treatment at these sites would not have a substantial impact on pharmaceutical loads entering STPs, and finally the environment. Overall, our approach demonstrates a unique opportunity to screen for pharmaceuticals used in hospitals and identifying priority pollutants in hospital wastewater explicitly accounting for site-specific conditions. Being based on consumption and loads discharged by hospitals into municipal wastewater, it is not limited by 1) the big effort to obtain representative samples from sewers, 2) the availability of sensitive chemical analysis or 3) a pre-selection of consumption data (e.g. consumption volume)