Efficiency of phosphate removal during anaerobic digestion of activated sludge by dosing iron(III) (2023)

Environmental management magazine

Part 193,

15 maja 2017 r

, pages 32-39

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Phosphate-Fe(II)precipitationby Fe(III) reduction duringanaerobic digestionexcessactivated sludgewas tested for phosphorus removal and the possibility of its recovery. Experiments were carried out with three Fe(III) sources at 35°C and 55°C. The results show that ferrihydrite-Fe(III) was effectively reduced during the anaerobic processsludge digestionby 63% and 96% in mesophilic and thermophilic conditions, respectively. While FeCl3-Fe(III) was only mesophilically reducible, and hematite-Fe(III) reduction was imperceptible at both temperatures. Efficient precipitationlong livenot observed, although high values ​​of the saturation index were achieved, e.g. >14 (reduction in activity was not taken into account). This reveals the complexity of vivianite precipitation in anaerobic digestion systems; For example, Fe(II) complex formation and organic interference could not be ignored. WITHferrihydriteWhen adjusted at Fe/TP 1.5, methane production from sludge digestion was reduced by 35.1% at 35°C and remained unchanged when the fermentation temperature was increased to 55°C. But acidic FeCl3seriously inhibited the production of methane and, consequently, the degradation of sludge biomass.


Eutrophication of aquatic environments causes undesirable growth of phototrophs, depletion of dissolved oxygen and the appearance of unpleasant odours. There are reports that phosphorus even at low concentrations of 0.02-0.3 mg/l can stimulate the growth of algae in lakes and rivers (Schindler, 1977). In many freshwater ecosystems, phosphorus is often a limiting factor in the occurrence of eutrophication (Correll, 1999, Teikari et al., 2015). The content of total phosphorus in raw sewage ranges from 10 to 15 mg/L, of which 50-70% is in the form of orthophosphates (Blackall et al., 2002). Many countries have strict rules on upper limits for phosphate discharges. The USEPA recommends limits of 0.05 and 0.1 mg/L for total phosphorus in streams entering lakes and flowing waters, respectively (The Cadmus Group, 2009).

To reduce the load of phosphorus (mainly phosphates) to receiving water bodies, phosphate removal technologies, including chemical precipitation and assisted biological phosphorus removal (EBPR) have been used in municipal wastewater treatment plants (WWTP) (Huang et al., 2015, Mao et al., 2014 , Morse et al., 1998, Park et al., 2017, Tervahauta et al., 2014). Biological methods reduce the phosphate content of wastewater by transferring the phosphate to the sludge phase. However, the resulting phosphate-rich sludge and sludge fermentation and dewatering effluents require additional treatment (often by chemical methods) to ensure that the biotreatment system does not deliver excess phosphate to the aeration tank. Chemical precipitation of phosphates is based on the formation of insoluble phosphates, for example calcium, iron and aluminum salts. The process is simple and reliable, but will significantly increase the cost of reagents (Gong and Zhao, 2014). As phosphorus deficiency has been identified, and in order to gain the benefits of phosphate removal, crystallization of phosphate for recovery becomes an essential step (Koppelaar and Weikard, 2013). Recent studies show that the crystallization of phosphate from digested sludge also improves sludge dewaterability, which reduces sludge disposal costs (Bergmans, 2011).

So far, the crystallization of phosphates from waste streams has mainly been studied and applied through the formation of struvite (ammonium magnesium phosphate, MAP) and hydroxyapatite (calcium phosphate, HAP) (Ye et al., 2017). Some fully operational facilities have also been developed and are operating in Europe, North America and Japan (Desmidt et al., 2015, Doyle and Parsons, 2002). However, for certain specific conditions, lower costs and/or better products, other precipitation/crystallization routes are needed. Invanov and colleagues found that phosphorus from wastewater was effectively removed (up to 90%) with ferrophosphate (FeHPO4) precipitation caused by the bioreduction of ferric ions (Ivanov et al., 2005, Ivanov et al., 2009). Chen et al. (2015) developed vivianite ([Fe3(NA4)2eight o'clock2O]) precipitation-based phosphate removal process in septic systems where alternating anaerobic and aerobic conditions for EBPR are not available. In fact, a significant proportion of phosphate in natural environments under highly reducing conditions, e.g.).

Recently, Cheng et al. (2015) studied phosphate deposition during anaerobic digestion of sludge with iron(III) addition and found that Fe(III) bioreduction, viviaite precipitation and methane production can occur in one system. This provides a new alternative method of phosphate recovery and can significantly reduce reagent costs, as the quality requirements of the iron source for Fe(II) production are much lower than those of reagents in other processes, e.g. Mg2+for MAP crystallization (Ivanov et al., 2009). Vivianite crystallization is also more competitive for iron ions than siderite (FeCO3), which reduces the need for an iron source to the system or skips the degassing step to remove bicarbonate as an undesirable iron sink (Saidou et al., 2009). However, compared to MAP or HAP crystallization, sufficient insight is lacking in the crystallization of vivianite-based phosphates in a wastewater treatment system. For example, the bioreduction of Fe(III) to vivianite formation in an anaerobic sludge digester was practically unclear. The crystallization of vivianite in a real sewage sludge system also requires additional information.

The aim of this work was to investigate the efficiency of phosphate removal in an iron(III) modified anaerobic digester by promoting the precipitation of wiviaite. Sludge fermentation was carried out under mesophilic and thermophilic conditions with three different iron(III) sources to study the relationship between phosphate removal and Fe(III) reduction.

Fragment section


Excess activated sludge was obtained from the return sludge line at Gaobeidian WWTP (Beijing, China). Anaerobic-anaerobic-aerobic (A2O) systems were used in this plant for improved nutrient removal with a sludge retention time of 20 days. The sludge was concentrated by gravity at room temperature for 24 hours before use. The characteristics of the thickened sludge are presented in Table 1.

Sources of iron(III).

Iron hydrate (α-FeOOH), iser(III) chloride (FeCl3) i hematyt (Fe2O3) were used as three forms of Fe(III) sources. Iron

Iron(III) reduction.

The rate of reductive dissolution of Fe(III) (by ascorbic acid) was reported to decrease with increasing crystallinity of the Fe(III)-containing mineral (Larsen and Postma, 2001). In Fig. 1a levels of aqueous Fe(II) in FeCl3and the ferrihydrite-modified vials grew in 9-12 days under mesophilic conditions with final values ​​of 81% and 54% of the total Fe in the system. This shows that the reduction of these two Fe(III) sources was effective in anoxic sludge


Enhancement of vivianite formation in anaerobic sludge digesters may be a new promising alternative for the removal and recovery of phosphorus from wastewater, but the process is not well understood, including the appropriate Fe sources and fermentation conditions. The results of these studies indicate that during the anaerobic digestion of excess activated sludge with Fe(III) corrections at Fe/TP equal to 1.5, ferrihydrite-Fe(III) can be effectively reduced both in mesophilic and thermophilic conditions by 63%

(Video) Wastewater Treatment - Nutrient Removal Intro


This work was supported byBeijing project of young elite higher education teachers(From home.YETP0772). The authors want tooBasic Research Funds for Central Universities(From home.2016ZCQ03) iNational Life Sciences Foundation of China(From home.51478040) for financial support.


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      Iron is an important element for modern wastewater treatment, including the removal of phosphorus from wastewater. However, recovering phosphorus from sewage sludge containing iron and phosphorus, without incineration, is not yet economically viable. We believe that advancing our understanding of iron and phosphorus speciation in sewage sludge can help identify new pathways for phosphorus recovery. Surpluses and digestate from two sewage treatment plants were studied. Plants relied solely on iron-based phosphorus removal or biological phosphorus removal assisted by iron dosing. Mössbauer spectroscopy showed that vivianite and pyrite were the dominant iron compounds in excess sediments and anaerobic digested sediments at both plants. Mössbauer and XRD spectroscopy suggest that vivianite-bound phosphorus comprised between 10 and 30% (in plants that rely primarily on biological removal) and between 40 and 50% of the total phosphorus (in plants that rely on iron-based phosphorus removal) . Furthermore, Mössbauer spectroscopy showed that none of the samples contained a significant amount of Fe(III), although there were aeration steps and although Fe(III) was dosed in addition to Fe(II). We hypothesize that the chemical/microbial reduction of Fe(III) in treatment lines is relatively rapid and causes the formation of vivianites. Once formed, vivianite can withstand oxygenated treatment zones due to slow oxidation kinetics and oxygen diffusion limitations in the silt flocs. These results indicate that vivianite is the most important iron phosphorus compound in sewage treatment plants with a moderate iron dosage. We hypothesize that vivianite predominates in most plants where iron is dosed to remove phosphorus, which may provide new pathways for phosphorus recovery.

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      Recently, more and more attention is paid to the strong oxidizing properties of the newly prepared potassium ferrate (NAPF) in the sludge reduction process, while less attention is paid to the conversion of phosphorus in this process. The feasibility of phosphorus migration and transformation during excess sludge reduction pre-treatment by NAPF pre-oxidation in combination with anaerobic digestion was investigated. After pre-treatment with NAPF 70 mg/g slurry and 16 days of anaerobic digestion, the volatile solids in the solid phase were reduced by 44.2% and a lot of organic matter was released in the liquid phase and then degraded during digestion by native microorganisms. During the process of pre-oxidation of sludge, in the form of PO43-, with a peak of 100 mg/l. During anaerobic fermentation, Fe3+in the liquid phase it was gradually reduced to Fe2+, and then formed Fe2+-NO43-complex crystals and migrated back to the solid phase. PO concentration43-decreased to 17.08 ± 1.1 mg/L in the liquid phase after anaerobic digestion. Finally, phosphorus in Fe2+-NO43-the compound accounts for 80% of the total phosphorus in the solid phase. A large number of viviaite crystals have been observed in the sediment. Therefore, this technology not only effectively reduces sludge, but also increases the PO content43-in the precipitate in the form of vivianite.

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      The paper presents an innovative method of recovering phosphorus (P) in the form of wiviaite from activated sludge (WAS) waste by optimizing the iron dose and pH value during anaerobic digestion (AF). The optimal conditions for the formation of viviaite were in the pH range of 6.0–9.0 with the initial PO43->5mg/L and Fe/P molar ratio of 1.5. Specifically, FeCl3showed superiority over ZVI in terms of joint issue of Fe2+not after43-during the fermentation of WAS, especially under acidic conditions. FeCl3dosing at pH 3.0 can contribute to 78.81% Fe2+release and 85.69% of total PO43-liberation from YOU. They were finally recovered as high purity viviaite (93.67%).Clostridiaceae(40.25%) was the dominant bacterium in FeCl3-pH3 reactors that played a key role in triggering the reduction of non-Fe-like iron2+creation. Therefore, recovering P as wiwiaite from WAS fermentation may be a promising and highly valuable approach to mitigate the P crisis.

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      Vivianite biosynthesis from microbial extracellular electron transfer and environmental applications

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      Vivianet (Fot3(NA4)2eight o'clock2O) is a common hydrated ferrous phosphate that is often found under reducing conditions, especially in an anaerobic non-sulfide environment containing high concentrations of iron (Fe2+) nie ortofosforan (PO43-). Vivianite is an important product of dissimilar iron reduction and a promising way to recover phosphorus from wastewater. Its formation is closely related to extracellular electron transfer (EET), a key mechanism of microbial respiration and a key explanation for the reduction of metal oxides in soils and sediments. Despite its natural ubiquity, easy availability and attractive economic value, vivianite's utility value has not received much attention. This review presents the characterization, occurrence, and biosynthesis of vivianite from microbial EET and systematically analyzes the environmental value of vivianite, including heavy metal (HM) immobilization, carbon tetrachloride (CT) dechlorination, sedimentary phosphorus sequestration, and eutrophication mitigation. In addition, the possible functions of a slow-release fertilizer are also discussed. Vivian is generally not expected to make a major contribution to future scientific research, especially addressing environmental issues. Overcoming the misunderstanding and certain technical limitations will be beneficial for the further environmental application of vivianite.

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      Iron-air fuel cell system for simultaneous removal of phosphorus and recovery of resources in the form of wiviait and power generation in wastewater treatment: sustainable phosphorus technology

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      Recovering phosphorus (P) from industrial wastewater solves both P deficiency and P contamination. Sequential iron-air fuel cell launched to recover P from synthetic wastewater containing 0.6 g-P/L Na2HPO4. In the cell, ferric iron passes from the iron anode into the fluid, precipitating soluble P and forming vivianite. Electrons travel from the iron anode to the air cathode through an external circuit to generate energy. Within 3 months of continuous use, the P removal efficiency steadily reached about 97.6%, and the average output voltage of the cell was 404 mV. After prolonged use, a deterioration in the performance of the iron fuel cell was observed due to the passivation of the electrode caused by the accumulation of P deposit on the surface of the iron anode. The deposit layer on the iron anode hindered, but did not block, the transfer of iron(II) mass to the anode fluid. The cell continued to operate with a 25% drop in output voltage, an 86% drop in current density, an 87% drop in power density, and a 9-fold increase in internal resistance. Further XRD, FITR and Mössbauer analyzes showed that vivianite was the main component of both deposits on the surface of the iron anode and at the bottom of the anode chamber with 66% and 30% content, respectively. Vivianite on the iron anode surface was preferred due to its higher content for P recovery. thus to the sustainable development of wastewater treatment.

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      Potentials and challenges of recovering phosphorus as wiviaite from wastewater: an overview

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      Due to the scarcity of phosphorus resources and the limitations of existing phosphorus recovery methods, the recovery of phosphorus in the form of vivianite is of great interest due to its natural ubiquity, ready availability and predictable economic value. This review systematically summarizes the chemistry of vivianite, including material characteristics, formation process, and influencing factors. In addition, the potential for the recovery of phosphorus as wiwiaite from wastewater was also extensively investigated from the point of view of economic value and technical feasibility. In general, this method is theoretically and practically feasible and provides some additional benefits in wastewater treatment plants. However, insufficient understanding of vivianite recovery in wastewater/sludge has been delaying the development and exploration of such a sophisticated approach. Further research and support in various fields would facilitate the refinement of this technique in the future.

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    (Video) Webcast of the Month: Process Control for Biological Nutrient Removal


    What is the percentage of sludge in phosphorus removal by chemical precipitation * 10% 20% 30% 40%? ›

    What is the percentage of sludge in phosphorus removal by chemical precipitation? Explanation: Phosphate removal is currently achieved largely by chemical precipitation, which is expensive and causes an increase in sludge volume by up to 40%.

    How phosphorus is removed in activated sludge? ›

    Chemical treatment for phosphorus removal involves the addition of metal salts to react with soluble phosphate to form solid precipitates that are removed by solids separation processes including clarification and filtration.

    Which method is generally used for removal of phosphate? ›

    Phosphate removal is currently achieved largely by chemical precipitation, which is expensive and causes an increase of sludge volume by up to 40%. An alternative is the biological phosphate removal (BPR).

    What is the effect of phosphorus on anaerobic digestion? ›

    High concentrations of soluble PO4-P (> 250 mg/l) were found to have a retarding effect on anaerobic digestion, reducing the rate of volatile solids digestion and methane production in comparison to control digesters.

    How does ferric chloride remove phosphorus? ›

    The added salt (e.g., trivalent metal salts, ferric chloride) precipitates P in the wastewater and resulting solids residuals are removed either by settling under gravity or by filtration.

    What is the removal rate of activated sludge? ›

    The activated sludge process generally removes more BOD than the trickling filter. Typically the trickling filter will remove 80 to 85% of the influent BOD. The activated sludge process will often remove 85 to 95% of the BOD from the aeration influent.

    What percentage should sludge be removed? ›

    Effective removal of sludge is very important for the efficient operation of clarifiers. With a raw water having suspended solids less than about 250 mg/l (most waters used in many countries fall in this category) the sludge volume to be removed from the tank should not exceed about 2.5% of inflow.

    What is the strategy for biologically removing phosphate from wastewater? ›

    The basic principle of biological phosphorus removal is to expose bacteria to alternating anaerobic and aerobic conditions to promote “luxury uptake” of phosphorus. Under anoxic to anaerobic conditions, phosphorus accumulating organisms (PAO) have the ability to take in organic substrate.

    What coagulant for phosphorus removal? ›

    Coagulation of phosphate in the anaerobic sludge can be accomplished by establishing a coagulation process after the digestion stage. The effective coagulants may include Al-based ones, such as alum and poly-aluminum chloride (PAC), and Fe-based ones, such as FeCl2 and FeCl3 [10].

    How much phosphorus is in sewage sludge? ›

    Sewage sludge has a high phosphorus content, approximately 8% (w/w), which makes it a potential source of the P nutrient (Biswas et al., 2009).

    What is the removal rate of phosphate? ›

    Phosphate adsorption efficiency was reported to be 80–90% at room temperature at an initial concentration of 30–100 mg L1. The percentage of phosphate removal decreases with an increase in pH.

    What is the strongest phosphate remover? ›

    Description. SeaKlear Phosphater Remover Commercial Strength is the industries most reliable and strongest phosphate remover.

    What are two major drawbacks of anaerobic digestion? ›

    The disadvantages of wet anaerobic digestion are low processing capacity, high operating costs; high requirements for the pretreatment of food waste and the ammonia nitrogen; salts are easy to inhibit wet anaerobic digestion. There is a risk of secondary pollution that is difficult to handle biogas residue.

    What is the effectiveness of anaerobic digestion? ›

    Anaerobic digesters can destroy more than 90 percent of disease-causing bacteria that might otherwise enter surface waters and pose a risk to human and animal health.

    What are the two main results of anaerobic digestion process? ›

    Anaerobic digestion produces two valuable outputs: biogas and digestate.

    How does iron affect phosphate? ›

    Iron (ferric carboxymaltose) infusion therapy is used to treat severe iron deficiency which is not responding to the first-line oral iron therapy. However, it can also cause severe renal wasting of phosphate resulting in severe hypophosphataemia in some patients.

    What is the removal efficiency of ferric chloride? ›

    Increasing the ferric chloride dose over 0.4 g/L exerts adverse effect in the coagulation efficiency. Therefore, the optimum ferric chloride dose that improved COD removal will not exceed 0.1 g/L.

    How much amount of phosphorus is removed by secondary treatment? ›

    Therefore, primary and secondary wastewater treatment can removes about 20-30% of phosphorus, and phosphorus content in pre-treated water is high above standard regulated limits.

    How much amount of sludge is reduced in anaerobic digestion of sludge? ›

    Anaerobic digestion (AD) is generated by biogas and it can be used as fuel to generate heat and electricity at STP [13]. Moreover, AD can reduce the organic matter content of sludge by 40 and 50% [14] which leads to a significant reduction in the final sludge volume [15].

    What is mostly activated sludge process used to remove? ›

    The activated sludge process is a means of treating both municipal and industrial wastewater. The activated sludge process is a multi-chamber reactor unit that uses highly concentrated microorganisms to degrade organics and remove nutrients from wastewater, producing quality effluent.

    How do you calculate removal efficiency? ›

    We have a formula that says efficiency or removal efficiency equals what's coming in minus what's coming out then you divide that by what's coming in and then we multiply by a 100 to convert the decimal to a percent.

    What is the density of 1% sludge? ›

    The average density of water sludge solids is 1400 kg/m3 and the density of water is 1000 kg/m3.
    Volume of sludge.
    Sludge solids content %kg sludge per kg dry solidsm3 sludge per tonne dry solids
    4 more rows

    How do you calculate sludge rate? ›

    Sludge volume index ( SVI ) is calculated by dividing the settleability by the MLSS concentration. The SVI is always expressed in mL/g.

    What is biologically enhanced phosphorus removal system? ›

    Enhanced biological phosphorus removal (EBPR) is a wastewater treatment configuration applied to activated sludge systems for the removal of phosphate. It is based on the cultivation of special polyphosphate-accumulating organisms (PAO) in the anaerobic tank prior to the aeration tank.

    What is the enhanced biological phosphorus removal mechanism? ›

    In enhanced biological phosphorus removal (EBPR) process, P is removed from wastewater and accumulated to waste activated sludge (WAS) by polyphosphate accumulating microorganisms (PAOs). Efficiently treated P-enriched biomass can be further used as a fertilizer directly or indirectly.

    What is the mechanism of phosphate removal? ›

    The phosphate removal mechanism is a complex process including crystallization via the interaction between Ca2 + and PO43 ; formation of precipitates of Ca2 +, Al3 +, and PO43 ; and adsorption of PO43 on some recalcitrant oxides composed of Si/Al/Fe.

    How is phosphorus removed from molten iron? ›

    The method entails melting a prepared slag in a separate furnace and tapping into the ladle before steel tapping. The induction furnace steel is then tapped onto this slag. The rapid mixing of the slag with the steel during tapping completes the removal of phosphorus even while tapping is going on.

    What blocks phosphorus absorption? ›

    Calcium from foods and supplements can bind to some of the phosphorus in foods and prevent its absorption [1,17].

    What is the effluent standard for phosphorus? ›

    Although there is no actual phosphorus discharge limit for wastewater into general water bodies, it is generally now considered that for a water body to actually achieve 'good' status under the WFD, a limit as low as 0.1 mg/l may be necessary.

    How much phosphorus is in dried sludge? ›

    The percent content of phosphorus in dry mass of sludge differs from that in ash after combustion: dewatered sewage sludge contains about 2.6–3.4% of phosphorus7,8, whereas the ash after combustion contains 5.9–13.4%9,10.

    Why is phosphorus in wastewater problematic? ›

    Too much phosphorus can cause increased growth of algae and large aquatic plants, which can result in decreased levels of dissolved oxygen– a process called eutrophication. High levels of phosphorus can also lead to algae blooms that produce algal toxins which can be harmful to human and animal health.

    How long does phosphate remover take to work? ›

    1. Pour phosphate remover into skimmer. 2. Run filter system for 24 hours (water will turn cloudy but will clear on its own, but may take a week to clear completely; but is still swimmable).

    What is the max infusion rate of phosphate? ›

    Central administration: o The maximum recommended concentration is phosphorus 18 mmol/100 mL (potassium 28.2 mEq/100 mL). o The maximum recommended infusion rate is approximately phosphorus 15 mmol/hour (potassium 23.5 mEq/hour).

    What is the maximum phosphate replacement? ›

    Administer over 2 to 3 hours (maximum rate 0.2mmol/kg/hour).

    Will phosphate remover lower pH? ›

    Conventional phosphorus removal using ferric or aluminum coagulants can make your pH balance even more difficult to maintain. These chemicals are extremely acidic, having a very low pH of 1.5 to 2.2.

    Can aeration remove phosphate? ›

    Under aerobic conditions the phosphate present in the solution was only partially taken up by the biomass resulting in only 24% phosphorus-removal efficiency. Within a day the BPR had fully recovered. The total oxygen consumption by the biomass was 84 mg-O2/litre.

    Do you really need phosphate remover? ›

    The fact is that you don't really need phosphate remover. If you want to avoid algae, keep your pool clean and maintain the chemical levels in your pool water. Phosphates on their own are not toxic, but some of the chemicals used to remove them are.

    What percent of the phosphorus is removed? ›

    Some phosphorus is removed as the wastewater flows through the septic tank. Some studies have estimated that as much as 20 to 30 percent of phosphorus becomes part of the settled solids in the septic tank.

    What is the percentage of sludge in wastewater? ›

    The solids content of mechanically dewatered sludge typically ranges from 20 to 45 percent solids by weight; most processes produce con-centrations of solids at the lower end of that range. Sludge conditioning processes do not, in and of themselves, reduce the water content of sludge.


    1. ClariPhos Rare Earth Coagulant: Remove more phosphorus, produce less sludge.
    (Bishop Water)
    2. Anaerobic Testing, Treatment & New Products
    (Aquafix, Inc)
    3. Wastewater treatment plants and anaerobic digestion
    (Stephanie Lansing)
    4. Calix Limited Webinar Series - Methods of Phosphate Removal in Wastewater
    (Calix Limited)
    5. Nitrogen Removal
    (Australian Meat Processor Corporation)
    6. Webinar Recording: Methods for Removing Phosphorus from Wastewater
    (Bio Microbics)


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