Management of Water Treatment Sludge

Residual management

Management of residuals

It is estimated that over 100,000 tons of water treatment sludge produced every year as the waste of conventional potable water treatment. This quantity is predicted to continually rise to cope with the increasing potable water demand as a result of population growth and urbanization, making the proper management and disposal of residual sludge more imperative than ever(Babatunde and Zhao, 2007).To address this issue, Ahmand presented a comprehensive review on a great range of papers to demonstrate various approaches and strategies for sustainable reuse of WTS through what is called 3R concept, namely recovery, recycling and reuse. And hence, hopefully, to provide multiple ways to maximise the value of sludge while minimising its negative impacts on the environment(Ahmad,2016).

The properties and composition of WTS varies greatly due to the different methods used to treat raw water of various qualities. During the flocculation unit of the water treatment process, coagulants are added into the raw water to promote the aggregation and settlement of dispersed and colloidal particles, making it a considerable contributor of the sludge. Therefore, WTS is often classified into two categories, which are Al-based and Fe-based sludges ( known as alum and ferric WTS respectively), as aluminium and ferrous salts are two of the most used coagulants. Previous studies have found that the WTS is presenting in a amorphous structure with highly porous surface when observed under a scanning electron microscope, imparting its ability of absorbing majority of fine particles(Yang,2008)

Apart from the physicochemical characteristics, several studies were conducted to determine the potential toxicity of both the organic and inorganic contaminants that are presented in WTS. It is worth to note that although alum sludge has the potential to affect algal growth in the receiving water, it is less toxic than ferric sludge in terms of long-term exposure. In contrast with the high concentration of heavy metals in the treated sludge, such as lead, cadmium, zinc and copper, the leaching of heavy metals is far lower than the maximum acceptable level. Therefore, WTS is considered to be non-hazardous and thus can be used in agriculture, horticulture and other land-based applications (Chiang,2009).

The porous sludge surface and non-hazardous properties, therefore, allow the further utilisation of WTS to handle the waste in a more cost-effective, eco-friendly manner. A great range of WTS reuse options that were developed throughout the world is provided and discussed in details in Ahmad’s paper. As a whole, the reuse options can be concluded into three categories, which are the recovery and reuse of WTS in wastewater treatment plants, utilisation of WTS as a raw material in civil engineering applications, and potential reuse of WTS in agricultural practices and other land-based uses. The rest of this document intends to illustrate some of the key sludge management approaches with their advantages and drawbacks.

1. Utilisation of WTS in wastewater treatment

 

Plenty of past studies have proved the viability of recovering coagulants from water treatment sludge and reuse it to remove impurities such as turbidity, BOD, COD, suspended solids and phosphate from wastewater. Various techniques are available to recover iron and aluminium salt from WTS quite effectively. For example, the recovery of coagulants from WTS can be achieved through acidification at the optimal pH condition of 2.5. Furthermore, it is suggested by Keeley that Donnan dialysis membrane is the most effective way that is able to selectively recover over 70% of alum from WTS without any undesired substances. Even though the techniques and efficiency of coagulants recovery requires further development, its benefits in terms of coagulant dose saving, sludge volume reduction and cost reduction of final disposal have shown the phosphorus (Xu,2009).

Apart from that, the study also shows that WTS that is rich in iron can be reused directly as coagulants to treat vegetable refinery wastewater and is able to achieve high removal of oil, grease, COD and TSS under certain pH and dose ranges. It is highlighted by the author that the combined use of alum sludge with fresh alum shows even higher removal efficiency than fresh alum alone.

Another reuse option is to apply WTS to remove contaminants and heavy metals from wastewater through the adsorption mechanism(Yang,2014). In this case, WTS acts as an effective adsorbent and able to remove contaminants such as phosphorus, arsenate, mercury and lead from the wastewater. Studies found that the effectiveness of this contaminant adsorption is highly affected by pH and its performance is better under acidic condition. For the removal of phosphorus specifically, the efficiency is being optimised to achieve 100% removal for both organic and inorganic phosphorus at the pH range between 4-6.  Ferric sludge is considered as a stronger adsorbent than alum sludge as for its higher porosity.

WTS can also be used as a replacement of traditional soils and gravels as substrates of constructed wetlands. Constructed wetlands are widely used, cost and energy efficient technique nowadays to naturally remove pollutants by infiltration mechanism. There is one drawback of CWs which is the low removal efficiency of nutrients such as nitrogen and phosphorus. Thus, environmental engineers used WTS, especially alum sludge as the wetland media and found a significant increase in P and N removal. However, one problem with this strategy is that although Al-based sludge presents great capacity to get rid of organic matter,  ammoniacal nitrogen, its ability to remove overall TN (total nitrogen) is not as satisfying, which might be due to the lack of denitrification caused by limited influent carbon source.

This problem could be solved by better aeration system using what is called a step- feeding strategy to enhance the influent carbon source, as reported by Hu. (Hu,2012)

Last utilisation of WTS is to promote the treatability of sewage sludge in co-conditioning and dewatering process. Significant improvement in dewaterability and settleability are found when incorporating alum sludge with the waste activated sludge during the waste management process. In addition, alum sludge could also boost the removal efficiency of phosphate from the rejected water. Despite all advantages of this strategy, the main concern which may prevent the large-scale application is the potential high capital investment in the long distance of sludge transportation from a water treatment plant to a sewage treatment plant. 

2. Application of WTS in civil engineering scopes

Apart from utilisation in the wastewater treatment area, WTS can also be applied as raw materials to produce construction supplies, including cement, bricks, concretes and lightweight aggregates.

Cement produced using WTS as a substitution of traditional raw materials such as limestone, sand and clay, is found to have even higher compressive strength with similar hydration process and identification behaviour during the curing process. It’s notable that the overall strength is quite sensitive to the percentage of sludge that is mixed in and 28 days strength reaches the maximum at 5.5% and start to decrease afterwards. However, it is reported that contaminants such as phosphorus or sulphate may cause the change in properties of produced cement and could potentially alter the settling time.

It is suggested by Wang that the sintered WTS shows considerably high strength and extremely low level of heavy metal leaching, making it a favourable raw material in manufacturing bricks and ceramics. It is achievable to meet different quality standards and regulations of bricks and ceramics of each country by controlling the incorporated sludge percentage and the sintering temperature(Ahmad,2016).

Sintering process refers to the compacting and forming a solid mass of material by heat or pressure without melting it to the point of liquefaction. This strategy also enables the potential of using WTS to lightweight aggregates (LWA) with the required density and water absorption property. LWA will be classified into either constructional or non-constructional concretes based on their specific properties. 

3. Agricultural practices and other land-based uses

The last common utilisation of WTS  is to apply in agricultural practice and other land based uses, it is based on the fact that soil is able to blend with the applied waste and get enriched without adversely affecting the soil quality. Typical agricultural reuse of WTS including soil pH adjustment, immobilisation of phosphorus in eutrophic lake recovery, pit filling and reclamation.

Depletion of soil quality might be caused by removal of base from the soil as a result of agricultural practices as well as the natural weathering. It is found that using lime soften sludge as a pH buffer for soil neutralization shows even higher effectiveness comparing to the conventionally used limestone. Also, the physicochemical properties of most of the sludge are found typically adequate for plant growth and hence could be a good source of topsoil during disturbed sites reclamation process.

Long term applications of fertilizers and biosolids often lead to higher level of nutrients than the crop requirements and thus could be transported to water bodies through runoff and/or erosion resulting eutrophication of surface waters.(Ahmand,2016) However, it is found that WTS amended soil can reduce plant-available P concentrations, and hence, decrease the amount of phosphorus available for off-site transport, which will eventually reduce P pollution and alleviate eutrophication in the receiving water body. Although there are concerns over the potential increase in Al concentration in soil when sludge is added, it is reported no notable elevated level on Al, probably because Al is mainly exist as insoluble Al(OH)3 in the soil. (Gallimore,1999)

Those are the option of sustainable reuse and management of residuals from water treatment plant that are discussed in the paper. Those options provide potential solutions to sludge disposal problem, with significant chemical savings through resource recovery and reuse, enhancing the treatment efficiency and also reducing the sludge volume. But it is highly unlikely that the wastewater treatment plant would be cited close to a WTP,  means the how to overcome the distance the transportation cost would be a problem.(Ahmand,2016)

Article 2: Arsenic waste management.

1.      Introduction

Over one hundred and forty million people around the world are affected by arsenic drinking water contamination. Many water treatment technologies have been developed to restrict negative health impacts due to exposure to arsenic contamination water. Ion exchange, coagulation, flocculation, adsorptive media filtration, as well as electrocoagulation and anaerobic removal with iron sulphides are used to treat arsenic contamination water. The arsenic drinking water guideline is lower from fifty to ten micrograms per litre by World Health Organization (WHO) in 1993 (Patterson, G. 2006). Following the stringent drinking water standard for arsenic, arsenic removal technologies in drinking water treatment processes has increased application. Appropriate methods for stabilizing arsenic wastes at disposal sites are required. In developed countries, the most common disposal environment is landfill disposal and stabilization. In developing countries, arsenic-bearing wastes are mixed with livestock waste, or mixed into building material such as bricks, disposed directly in surrounding ponds or on open fields with no monitoring. In this report, common arsenic-bearing wastes produced, waste disposal options, results of simulation experiments, common testing procedures used for managing arsenic-bearing wastes and suggestion of new directions to enhance adaptability of best practices for arsenic-bearing waste disposal are discussed (Tara et al. 2013).

2.      Arsenic-bearing waste

There are two most general inorganic dissolved forms which are arsenite, AsO₃^3‑, where arsenic has the oxidation number of positive three (As(III)), and AsO₄^3‑, arsenate,  with arsenic oxidation number of positive five (As(V)). In general, arsenic is more toxic for mammals in As(III) than in As(V). Arsenate can only precipitate in few amounts. Also, it is quite complicated by iron and aluminium hydroxide solids, while arsenic with oxidation sate II and III precipitate as reduced sulphides. Aqueous sulfidic arsenic species is significant in arsenic partitioning under sulfate reducing conditions. Another less common state of arsenic is arsine, AsH₃, with arsenic is in the −3 oxidation state (As(−III)). It is highly toxic and has high solubility in water. By oxidation, reduction, methylation and demethylation, abiotically and biotically (shown in Figure 1), arsenic species can be converted among various states. Expect for arsenic-bearing waste disposal, there are other factors affect successful adoption of arsenic removal technologies, including monitor effluent arsenic concentration, cost, and maintenance, etc.

Figure 1.

3.      Arsenic removal technologies

3.1 Oxidized arsenic-bearing wastes

Oxidized arsenic-bearing wastes are the most usual arsenic removal technologies that can remove arsenic as arsenate (As(V)) under oxic conditions. Ion exchange, adsorbent media filtration, coagulation and flocculation, and electrocoagulation are applied in oxidized arsenic-bearing wastes. Initially, arsenic presents as arsenite. And because of higher adsorption capacities on many adsorbents, arsenate is adsorbed to iron or metal acid oxides. pH and the concentrations of competing ions such as phosphate, silicate, natural organic matter, carbonate or bicarbonate can determine the effectiveness of arsenate adsorption.

3.2 Reduced arsenic-bearing wastes

Iron sulphides are produced abiotically or produced by sulfate-reducing bacteria or archaea. Then, arsenic can be sequestered under reducing conditions by coprecipitation or adsorption with these iron sulphides.  The presence of silica will not affect arsenic removal by iron sulphides, being a potential advantage in the presence of high silica concentrations. Under sulfate-reducing conditions, arsenic removal was limited not only by pyrite formation, but also the solubility of arsenic sulphides restrict the effectiveness of arsenic removal. By producing iron and arsenic sulphides in anaerobic active carbon fixed-bed bioreactor, arsenic is removed from drinking water. Although arsenic wastes are predicted to be more stable in reduced environments, the stability and potential for arsenic leaching have not been tested. In general, iron solids and iron sulphides are reduced by abiotic Arsenic III or II oxidation state, which is dependent on the type of solids and the pH. At low pH, slow precipitation of oxidized iron and fast dissolution of iron sulphides happen, which cause the release of arsenic in the aqueous phase.

4.      Arsenic waste disposal environments

There are quite a lot of disposal options of arsenic-bearing wastes, which includes landfills, stabilization, cow dung, passive aeration systems and directly soil disposal.

4.1 Landfill

In the U.S. and most of the developed countries, arsenic wastes are disposed of in landfills. There are four stages which are the initial aerobic stage, acidogenic stage, initial methanogenic stage and stable methanogenic stage. Firstly, in the presence of oxygen without hydrogen sulphides, it is a high chance that arsenic bound to oxidized iron or aluminium oxyhydroxide solids. Then, during the time of no oxygen and hydrogen sulphides present, arsenic is as arsenite form in the aqueous phase. Thirdly, when oxygen is absent and hydrogen sulphides are present, the precipitation of arsenic sulphides and iron sulphides remove arsenic from the aqueous phase. Microbes remove oxygen and resulting in anaerobic condition at the initial aerobic phase. The release of arsenic happens from microbial arsenic and iron reduction under reducing conditions. Under anaerobic conditions, fermenting bacteria provides acidic conditions. For the anaerobic stages, methane is produced by methanogenic archaea. To measure the methylated arsenic species, different techniques have been developed. Abiotic and biotic columns studies where ferric hydroxide and activated alumina waste were exposed to synthetic landfill leachate. That means leaching of arsenic appeared more frequently or rapidly. Also, leaching of arsenic is to a greater extent in the biotic columns (Cortinas et al, 2008).

4.2 Stabilization

The waste is stabilized and reduce the toxicity and mobility, through the addition of lime, iron-containing amendments, concrete, novel material such as polymeric matrices. After stabilization, arsenic wastes are disposed on soil, in Landfill or used in bricks (future research required) (Sullivan et al, 2010). It is primarily used in developed countries, and now gradually carried out in developing countries. pH, relative humidity, wetting and drying cycles are the factors affect leaching of arsenic from stabilized waste. Cost, arsenic stability, concrete strength, costs of waste transport, evaluation of long-term arsenic leaching potential should be considered in future studies.

4.3 Disposal with cow dung

Disposal with cow dung promotes microbial arsenic methylation to produce gaseous methylarsines. Quite a few studies proved that disposal with cow dung is less toxic to mammals than inorganic forms. For rural areas in developing countries, disposal with cow dung is recommended as an optimal disposal strategy. In some test, only a little of arsenic volatilized with the addition of cow dung. However, there are some tests do work well with this method. In another anaerobic incubation, organic food waste and sewage sludge are added and mixed with arsenic wastes. The mixture was then incubated in an anaerobic digester for 50 days. Up to 99% of arsenic removal is reported. Therefore, more simulation of actual conditions should be done in order to get good result of the arsenic removal. Moreover, the potential for arsenic release in the aqueous phase should also be evaluated, since the anaerobic digesters can promote volatilization of arsenic. Measurements of all arsenic phases should be included in future studies so that gaseous and aqueous emissions can be accurately reported. (Das et al, 2001)

4.4 Passive aeration disposal

Passive aeration disposal is applying a container that has slotted pipes running through the container to hold arsenic-bearing wastes, so air is sufficient to contact with the waste and helps maintain oxic condition. Arsenate will then stay bounding to the resins or iron hydroxides or aluminum hydroxide media. After that, there is one central location where the spent iron hydroxide or aluminum hydroxide based adsorbent media from the filters are collected and disposed. However, the effectiveness has not yet been monitored or studied. The location of pipes and vents are expected to be determined or experimentally verified. Further research should focus on the effects of changing environmental conditions, including redox and pH, as well as flooding and cycles of wetting and drying.

4.5 Pond disposal

The pond is the ultimate disposal location in some developing countries. When the filter is under maintenance, Arsenic-bearing iron hydroxides particles in slurries of water are disposed in ponds. Only 2 studies are about the pond disposal, which means it is not widely studied. The effectiveness, stability of waste and other considerations are not sure. Following studies should focus on a comprehensive understanding of ponds, as well as monitor ponds where arsenic-bearing waste is disposed.

4.6 Direct soil disposal

Direct soil disposal is commonly used in developing countries that have no access to engineered landfills. Few pieces of research exist, but it is recommended for waste generated by the SONO filter. Further analyses are required because crops and fish may uptake arsenic waste if using direct soil disposal. It will increase the potential arsenic exposure for people in arsenic affected areas and cause negative impacts on human health.

5.      Arsenic waste testing procedures

Arsenic waste testing procedures are to quantify arsenic leaching potential in disposal environments. There are many arsenic waste testing (shown in Table 1). Toxicity characteristic leaching procedure (TCLP) is the most commonly used test for U.S. environmental protection agency. Other tests are not widely for arsenic-bearing wastes, including the Japanese standard procedure for leaching test, and the Korean waste standard test.

5.1 extraction liquid

Extraction liquids is used in the tests. Acid-based extraction liquids are the most common used extraction liquid in arsenic waste testing procedures (presence of acid in landfills). For TCLP, acetic acid is used, while citric acid is used in Cal WET. For TALP, distilled water with nitric acid at pH 4 and 7 (mimic local rainwater) is used, while in SPLP test, nitric and sulfuric anitricids (mimic acid rain) are used in an appropriate concentration. Both TALP and TCLP give a similar result in rainwater, but in soil or pond disposal are tending to be more complex and include microbially mediated release the arsenic from wastes. In the pH-stat test, to keep extraction liquid at pH 4, distilled water is titrated with nitric acid and sodium hydroxide (lower result in arsenic release than TCLP) is the extraction liquid. What’s more, distilled water with nitric acid at pH 4 is used as extraction liquid in modified Dutch column test which gives higher result in arsenic leaching than UK Environment Agency test and ASTM test — for testing soil. Concentrated nitric and hydrochloric acids are heated as extraction liquid in Korean standard waste test. Distilled water without pH adjustment in DIN 38414 (similar result as TCLP) is not suitable for predicting leaching in disposal environment.

5.2 Test duration

The test duration of the different test is from 1hr to 13 weeks. When TCLP extraction has been tested for longer periods (up to 84 h), more arsenic leaching occurred. Better estimation the maximum leaching potential can be done by increasing the duration of the pH-stat test from 24 to 96 hours. Increasing the duration of these tests can generate more conservative predictions of long-term leaching. However, future work should consider actual leaching test measurements under field conditions.

5.3 Criticisms of testing procedures

One way to predict long-term leaching behavior under anaerobic disposal conditions found in landfills is using air as the headspace gas. The use of zero-headspace and nitrogen purged vessels should be considered in regulatory tests for better simulation of the reducing conditions.

6.      Conclusion

The potential risks for the release of arsenic to the environment and contamination of water and food should be monitored and controlled. More tests should be carried out to assess the stability of wastes under conditions with stimulated microbial activity. Wastes should be stored in environments with limited microbial activity, such as concrete stabilization, considering the ability of microbially mediated transformations of arsenic and iron. What’s more, changing conditions in landfills and other disposal environments over both time and space should be considered and explored. The characterization of chemical and microbial changes during disposal should be studied and developed. Last but not least, modifying short-term testing is important to represent the conditions of disposal environments, which gives higher accuracy in assessing the long-term leaching potential of wastes.

Article 3: Acceptability of land application of alim-based water treatment residuals

The growing world population comes with a consistently high demand for potable water. To meet such demand, water purification is necessary to produce reliable and safe drinking water. However, it is accompanied by the generation of waste treatment residuals, which would cause adverse environmental impacts if it is not disposed appropriately. Conventional methods of disposing WTR include discharging the residuals to natural water body, to sewer, to lagoon or landfilling. Although each of them has their own advantages, these disposal routes are not sustainable or cost effective. This encourages the exploration of alternative methods on disposing WTR in a more environmentally friendly and sustainable way. In this study, it showed multiple countries have implemented various WTR reuse methods, which include the use in wastewater treatment process, use as building/construction materials and land-based application. Among all the reuse methods, land application on residual products(biosolids) has been widely regarded throughout the EU as the Best Practicable Environmental Option (BPEO)(Zhao et al, 2018). However, the effectiveness of land application of WTR remains debatable as there are limited guidance and universal acceptance. Therefore, this study aims to analyse and determine the practicability of WTR application in agricultural land.

There is a doubt about the reliability of 32 papers they screened and reviewed as composition variation of the WTR may happened over a large time span. In the study, they took the average of WTR composition in a 10-year interval and compared with different interval(3 groups).

For organic matter, the result showed an increase in natural organic matter in surface water due to the climate change happened for the last 30 years. It may oppress water treatment works and result in high coagulant and organic matter content in WTR.

For nutrient contents in WTR, macronutrients varied inconsiderably from 1985 to 2017, keeping an average level of 5g/kg for nitrogen and 2.5g/kg for other macronutrients.

Regarding the micronutrients in WTR, the content of Manganese increased considerably over the years. Manganese plays an important role in several physiological process in plants, especially in photosynthesis process. With this Mn-rich WTR application in soil, soil with Mn-deficiency may be corrected, which helps plant growth. However, excessive introduction of Mn-rich WTR may lead to Mn phytotoxicity and hinder the plant growth.

For Heavy metals, most of the HM contents remain similar except Arsenic(As) and Chromium(Cr), which are markedly higher than the past years.(Wang et al, 2015)

Analysis of the effectiveness of WTR land application has been carried out in this study. Both positive and negative results had been recorded. Although most results appear to support the land application of WTR, the conclusion is still ambiguous and unable to provide direction for future application. Several assumptions have been made for this analysis. First, a conversion factor has been adopted to normalise all values in g/kg(Mukherjee et al, 2014). Second, positive result was defined as increasing vegetation dry mass while negative result was defined as decreasing vegetation dry mass. Overall, with varying parameters such as pH of soil and WTR, applied rate of WTR, scheme of WTR and background soil type, the governing factors for positive and negative results are still vague. Although a linear correlation between different parameters can not be converged, the general effects on different parameters can still be concluded.

For negative results, 9 out of 12 results were related to Phosphorus-deficiency while the other 3 results were related to Mn, Al³⁺ and Heavy metal phytotoxicity. All negative results recorded a decrease in Phosphorus content per dry mass of plants. In general, the results showed P-deficiency was definitely the reason for dry mass reduction with WTR application. The study also suggested applying WTR and P-fertilisers together is improper as the high P fixation capacity of WTR cannot mitigate the deficiency but unavoidably mask and reduce the positive reaction of fertilisers with the plant. Although 4 results showed an improvement of soil structure and an increment of water holding capacity, the deficiency strongly outweighed the soil physical improvement.(Codling et al, 2002)

For positive results, 10 out of 17 results were recorded with soil physicochemical properties improvement such as pH, electrical conductivity, water holding capacity, cation exchange capacity, organic matter content and soil aeration. Pore size distribution and aggregate stability were improved with the application of WTR, resulting in a better soil structure. These improvements increase the permeability of the soil, thus allowing roots penetration to hold on to the soil and protect the soil from erosion. However, similar to the negative results, most positive results has a Phosphorus content decrease even though increase in dry mass of plant has been recorded. The results showed the improvement of soil physicochemical properties neutralised the loss in P-content, thus recording a gain in biomass for plants.

Although the above analysis showed a general idea on how the positive and negative results behave, the governing factors for positive and negative results are still vague, which guideline for WTR land application can not be converged. Therefore, in the study, they tried to analyse the results and study the impact of different parameters to the soil. 4 general categories of parameter had been analysed, which are pH of WTR and soil, applied rate of WTR, applied scheme of WTR, soil types and quality.

For pH of WTR & soil and the applied rate of WTR, the study suggested zones can be defined for any combination of pH of soil and WTR and applied rate of WTR to be harmless to plant’s growth. A zone with the range of pH5-10, applied rate from 0-50g/kg has been defined as most positive results lied in this zone. In order to balance the effects of pH on Al³⁺, Heavy metals and Calcium, the pH of soil should be restrained to create a valley between Phosphorus fixation by iron and aluminium at low pH and by calcium at higher pH to prevent posing toxicity to plants and reducing nutrients available for plants’ growth. Therefore, a value of pH5 was concluded as a ration critical point to reduce the adverse impacts of WTR application. For applied rate of WTR, it is hard to define an upper limit based on the results as it is not an independent variable but a dependent variable, subjected to change according to other factors. Therefore, in the study, they took a upper limit of 50g/kg which included 90% of the positive results and a more conservative limit of 20g/kg which included 50% of the positive results. In general, soil pH affects the availability of different nutrients essential to plant growth and toxic elements. However, the optimum pH varies on which nutrients is the most abundance and toxic elements is the least present. As the optimum pH varies from plants to plants, it is difficult to state a explicit pH for WTR land application. They concluded that pH is not a predominant parameter deciding the result of WTR land application once the pH of soil is higher than 5.0.

For WTR applied scheme (surface applied or thoroughly mixed), 23 out of 29 results were conducted in the greenhouse while 6 was conducted on field-scale. Most results conducted in the greenhouse adopted a thoroughly mixed scheme while the other 6 adopted a surface applied scheme. 7 out of 8 results with surface applied scheme achieved a positive feedback to WTR amendment, while only 48% of the mixed scheme achieved a positive feedback to WTR. For this comparison, surface applied scheme is highly recommend as it is the only practical way to employ WTR on a field scale to mitigate the Phosphorus deficiency impacts.(Cox et al, 1997)

For soil types and quality, based on the results combining both positive and negative with different types of soil, they concluded that soil types are not a determining factor in regulating the response of plants’ growth. Soil quality was classified as normal and deteriorative soil based on their original quality. Results showed that fertilisers supplements is yet the most direct and instant way to improve the fertility of soil, which can turn soil into normal and favourable soil when added to deteriorative and normal soil respectively. However, with the addition of WTR, the strong Phosphorus adsorption property from WTR will decrease the fertility of the fertilised soil, which is actually hindering the growth of plant. Results with positive feedback also showed deteriorative soil without fertilisers supplement works the best for WTR to mitigate the harmful effects of soil for plants such as pH neutralisation and heavy metals immobilisation, which promotes the growth of plants. The above results showed WTR is highly recommended to employ onto deteriorative soil as an supplement and WTR should be avoided to apply together with fertilisers, which will nullify the positive effect of fertilisers.

Apart from the technical issues to determine the best and the most efficient way to apply WTR for agricultural purposes, legislation is also another big obstacle to tackle in order to implement WTR land application. Due to the fact that WTR land application is relatively new and debatable, laws of most countries haven’t explicitly stated and regulate the WTR application, which hinders the promotion on WTR recycle and reuse on land for agricultural purposes. With the increase of surface water turbidity and organic material due to climate change, the volumes of WTR will eventually increase to meet the demand of drinking water. However, different states have different legislation and policy on WTR in Australia, which slow down the progress of promoting WTR land application. For example, in Queensland, WTR is classified as a general waste and is usually stockpiled on-site or transported to landfill for disposal (GHD,2018) . With the landfill levy exempted from 2012, there is less incentive to encourage government to investigate a more sustainable way to recycle and reuse WTR. On the other hand, in South Australia, land application of WTR has been approved to grow plants that are not for human consumption (Verrelli,2008), which set a leading role in Australia to recycle the WTR and bring less adverse environmental impacts.

In conclusion, recycle and reuse of WTR offers a more sustainable and alternative way to minimise the negative environmental impacts WTR brings when comparing to landfilling. However, more research will be needed in order to understand the underlying potentials and dangers and regulate the use of WTR on agricultural use.

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