Chirag M. Mehta

Platforms for energy and nutrient recovery from domestic wastewater: A review

Alternative domestic wastewater treatment processes that recover energy and nutrients while achieving acceptable nutrient limits (<5mgNL(-1)) are a key challenge. Major drivers are value and availability of phosphorous, nitrogen, and potassium, and increasing energy costs. The two major platforms that can achieve this are (a) low energy mainline (LEM), with low strength anaerobic treatment, followed by mainline anaerobic nitrogen removal and chemical or adsorptive phosphorous removal and (b) partition-release-recover (PRR), in which carbon and nutrients are partitioned to solids through either heterotrophic or phototrophic microbes, followed by anaerobic digestion of these solids and recovery from the digestate. This paper reviews practical application of these processes, with a focus on energy costs. Compared to conventional processes which require 0.5kWhkL(-1) electricity (500mgCODL(-1) influent concentration), PRR requires only 0.05kWhkL(-1) electricity. LEM offers the possibility to recover 0.1kWhkL(-1) as electricity with net energy generation above 400mgCODL(-1)influent, while PRR becomes energy generating at >650mgCODL(-1). PRR offers the possibility for recovery of nitrogen and other nutrients (including potassium) through assimilative recovery. However, the energetic overhead of this is substantial, requiring 5kWhkgN(-1) as electricity, which compares to ammonia fixation costs. The lower energy costs, and near to market status of LEM treatment make it likely as a recovery platform in the shorter term, while ability to recover other elements such as nitrogen and potassium, as well as enhance favourability on concentrated wastewaters may enhance the desirability of partitioning in the longer term.

Technologies to Recover Nutrients from Waste Streams: A Critical Review

Technologies to recover nitrogen, phosphorus, and potassium from waste streams have undergone accelerated development in the past decade, predominantly due to a surge in fertilizer prices and stringent discharge limits on these nutrients. This review provides a critical state of art review of appropriate technologies which identifies research gaps, evaluates current and future potential for application of the respective technologies, and outlines paths and barriers for adoption of the nutrient recovery technologies. The different technologies can be broadly divided into the sequential categories of nutrient accumulation, followed by nutrient release, followed by nutrient extraction. Nutrient accumulation can be achieved via plants, microorganisms (algae and prokaryotic), and physicochemical mechanisms including chemical precipitation, membrane separation, sorption, and binding with magnetic particles. Nutrient release can occur by biochemical (anaerobic digestion and bioleaching) and thermochemical treatment. Nutrient extraction can occur via crystallization, gas-permeable membranes, liquid–gas stripping, and electrodialysis. These technologies were analyzed with respect to waste stream type, the product being recovered, and relative maturity. Recovery of nutrients in a concentrated form (e.g., the inorganic precipitate struvite) is seen as desirable because it would allow a wider range of options for eventual reuse with reduced pathogen risk and improved ease of transportation. Overall, there is a need to further develop technologies for nitrogen and potassium recovery and to integrate accumulation–release–extraction technologies to improve nutrient recovery efficiency. There is a need to apply, demonstrate, and prove the more recent and innovative technologies to move these beyond their current infancy. Lastly, there is a need to investigate and develop agriculture application of the recovered nutrient products. These advancements will reduce waterway and air pollution by redirecting nutrients from waste into recovered nutrient products that provides a long-term sustainable supply of nutrients and helps buffer nutrient price rises in the future. Graphical Abstract:

Nucleation and growth kinetics of struvite crystallization

Struvite crystallization technology is being widely applied in full-scale due to a surge in nutrient demand and phosphate price increases. Past investigations on struvite crystallization focused on process efficiency and thermodynamics, and less on kinetics, while mainly using fluidized bed type crystallizer. In this work, nucleation and growth kinetic data were measured using stirred vessel. The primary and secondary nucleation was measured in synthetic wastewater, and crystal growth in digested supernatant. The measured kinetic data was correlated with solution supersaturation. The dependence of growth rate on relative supersaturation in the digested was higher compared to synthetic wastewater. The crystal polymorph in synthetic wastewater and real wastewater was comparable. Products from the growth studies showed narrow size distribution and favorable separation characteristics. The secondary nucleation rate showed second order dependence on relative supersaturation. The nucleation induction time decreased with an increase in supersaturation ratio with a clear distinction between homogenous and heterogeneous primary nucleation.

Nutrient solubilization and its availability following anaerobic digestion

This study aims to investigate solubilization of elements (P, N, K, Ca and Mg) during anaerobic digestion (AD) of solid agriculture waste. It is important to maintain particularly phosphorous in the aqueous phase to be able to subsequently recover it in a concentrated form via crystallization. Batch AD was carried out at a mesophilic condition (37 °C) and pH 7.0 ± 0.2 on a variety of piggery and poultry solid waste streams. Less than 10% of the total P, Ca and Mg was in soluble form in the digestate. Most of the N and K remained soluble in the digestate. A bioavailability test (citric acid extraction) showed P, Ca and Mg in the digestate were totally available. Complete solubilization of P, Ca and Mg occurred below a threshold of pH 5.5. This indicates these nutrients were released during digestion, and then either bound to form inorganic compounds or adsorbed on solid surfaces in the digestate. These effects reduce the feasibility of post-digestion recovery of the nutrients via struvite crystallization. Strategies to improve nutrient solubilization and recovery during the AD include addition of complexing chemicals, operation at depressed pH, or otherwise modifying the operating conditions.

Growth Rate Kinetics for Struvite Crystallization

The crystallisation of struvite is one means of removing phosphate (and nitrogen) from nutrient-rich wastewater. As with all crystallisation processes, the growth rate of the crystals and its dependence on supersaturation is of considerable interest in designing processing equipment Modelling of struvite solubility using the computer package MINTEQA2 showed that struvite exhibits a minimum solubility in moderately alkaline conditions, with increasing solubility in acidic and strongly alkaline solutions, while temperature has only a minimal effect on struvite solubility. To measure the growth rate of struvite crystals, laboratory measurements were conducted in an isothermal batch I L stirred seeded crystallizer. Experiments were performed in aqueous solution at three pH levels (7.5, 8.0 and 8.5) and three temperatures (25, 35 and 40 degrees C) for similar initial ionic concentrations with phosphate being the limiting ion. These conditions were chosen to cover the pH values at which struvite readily crystallises without excessive adjustment of the wastewater pH, and at temperatures typical of many wastewaters. The increase in crystal size with time together with the decrease in the concentration of the ionic species in solution were followed. Rapid crystallisation kinetics were observed in the solution, with growth rates over 20 mu m/min at pH 8.5. A second order dependence of growth rate on supersaturation (as phosphate concentration) was observed, and the growth rate constant increased as pH increased but was insensitive to temperature over the range of conditions used. Results from a 200 L pilot-scale trial of struvite crystallisation at a local abattoir were consistent with the laboratory results.

Correlation of second virial coefficient with solubility for proteins in salt solutions

In this work, osmotic second virial coefficients (B22) were determined and correlated with the measured solubilities for the proteins, α‐amylase, ovalbumin, and lysozyme. The B22 values and solubilities were determined in similar solution conditions using two salts, sodium chloride and ammonium sulfate in an acidic pH range. An overall decrease in the solubility of the proteins (salting out) was observed at high concentrations of ammonium sulfate and sodium chloride solutions. However, for α‐amylase, salting‐in behavior was also observed in low concentration sodium chloride solutions. In ammonium sulfate solutions, the B22 are small and close to zero below 2.4 M. As the ammonium sulfate concentrations were further increased, B22 values decreased for all systems studied. The effect of sodium chloride on B22 varies with concentration, solution pH, and the type of protein studied. Theoretical models show a reasonable fit to the experimental derived data of B22 and solubility. B22 is also directly proportional to the logarithm of the solubility values for individual proteins in salt solutions, so the log‐linear empirical models developed in this work can also be used to rapidly predict solubility and B22 values for given protein–salt systems.

Recovery of energy and nutrient resources from cattle paunch waste using temperature phased anaerobic digestion

Cattle paunch is comprised of partially digested cattle feed, containing mainly grass and grain and is a major waste produced at cattle slaughterhouses contributing 20-30% of organic matter and 40-50% of P waste produced on-site. In this work, Temperature Phased Anaerobic Digestion (TPAD) and struvite crystallization processes were developed at pilot-scale to recover methane energy and nutrients from paunch solid waste. The TPAD plant achieved a maximum sustainable organic loading rate of 1-1.5kgCODm(-3)day(-1) using a feed solids concentration of approximately 3%; this loading rate was limited by plant engineering and not the biology of the process. Organic solids destruction (60%) and methane production (230LCH4kg(-1) VSfed) achieved in the plant were similar to levels predicted from laboratory biochemical methane potential (BMP) testing. Model based analysis identified no significant difference in batch laboratory parameters vs pilot-scale continuous parameters, and no change in speed or extent of degradation. However the TPAD process did result in a degree of process intensification with a high level of solids destruction at an average treatment time of 21days. Results from the pilot plant show that an integrated process enabled resource recovery at 7.8GJ/dry tonne paunch, 1.8kgP/dry tonne paunch and 1.0kgN/dry tonne paunch.

Nutrient recovery from wastewater through pilot scale electrodialysis

Nutrient recovery performance utilising an electrodialysis (ED) process was quantified in a 30-cell pair pilot reactor with a 7.2 m2 effective membrane area, utilising domestic anaerobic digester supernatant, which had been passed through a centrifuge as a feed source (centrate). A concentrated product (NH4-N 7100 ± 300 mg/L and K 2490 ± 40 mg/L) could be achieved by concentrating nutrient ions from the centrate wastewater dilute feed stream to the product stream using the ED process. The average total current efficiency for all major cations over the experimental period was 76 ± 2% (NH4-N transport 40%, K transport 14%). The electrode power consumption was 4.9 ± 1.5 kWh/kgN, averaged across the three replicate trials. This value is lower than competing technologies for NH4-N removal and production, and far lower than previous ED lab trials, demonstrating the importance of pilot testing. No significant variation in starting flux densities and cell resistance voltage for subsequent replicate treatments indicated effective cleaning procedures and operational sustainability at treatment durations of several days. This study demonstrates that ED is an economically promising technology for the recovery of nutrients from wastewater.

Nutrients in Australian agro-industrial residues: production, characteristics and mapping

ABSTRACT Australia is a net food exporter, but relies heavily on imported agricultural nutrients. There is scope to recycle nutrients from agro-industrial residues, but this has not been assessed in detail. The assessment reported identified significant potential for nitrogen, phosphorus and potassium to be recovered from different Australian agro-industrial residues. Suitable sources of residues were cattle feedlots, piggeries, poultry (layers), poultry (meat), sugar cane processing, meat processing, dairies, coal mining, milk processing, fish processing, urban waste and electricity generation. Significant nitrogen, phosphorus and potassium sources were municipal waste, sugarcane processing, cattle feedlots and sewage wastewater. A total of 246 kt N, 88 kt P, and 359 kt K was produced as residues in 2010, representing 23% of N and P, and >100% of K in relation to the annual national agricultural consumption. Australian agro-industrial nutrient source–sink maps revealed that the best potential residual nutrients sources were intensive livestock facilities, which are generally located within 200 km of grain-producing areas. However, due to existing markets or inexpensive disposal sinks in Australia, there is a limited availability of these residues for practical use. This may change as the value and/or demand of nutrients increases in the future.

Towards a renewable future: assessing resource recovery as a viable treatment alternative state of the science and market assessment

This report presents a review of extractive nutrient recovery technologies with an emphasis on bridging the knowledge gap faced by utilities when considering nutrient recovery for nutrient management. The report provides a framework for selecting a nutrient recovery option and, depending on the conditions at a water resource recovery facility, establishes whether keeping phosphorus in biosolids is more or less beneficial than concentrating it in an inorganic phase such as struvite.