Ferric coagulant recovered from coagulation sludge and its recycle in chemically enhanced primary treatment

Evaluation of combined thermo-chemical processes for the treatment of landfill leachate using virgin and recovered FeCl3 coagulants

Sludge resulting from treatment of municipal waste landfill leachate contains suitable cationic substances such as Fe-based recovered coagulants which, if not recovered, can cause environmental problems. The present study aimed to maximise coagulant recoverability and investigate its potential reuse for the treatment of municipal waste landfill leachate. The study focused on establishing (i) the effect of mineral acids on leaching of Fe, (ii) the % of maximum recovery of Fe coagulant, (iii) the impact of ultrasound on recovery, and (iv) effectiveness of recovered coagulant when reused in coagulation-flocculation treatment of landfill leachate. Sulfuric acid outran hydrochloric acid in performance, with the acid leaching process leading to the recovery of 70.12% of Fe (acid concentration = 3.80 M, solid-to-liquid ratio = 8%, and heating time = 5 h). Subsequently, a developed acid leaching process was tested, which results showed that the highest rate of Fe recovery occurred without ultrasound treatment, meaning the use of it could reduce the recovery rate due to the increase in the iron (III) oxide-hydroxide [Fe(OH)3] sedimentation. Comparative experiments were undertaken with the recovered and virgin coagulants. These revealed that Fe-based recovered coagulant led to the 60.21% and 91.40% removal of COD and total suspended solid respectively, while the values of the COD and total suspended solid removal with the virgin FeCl3 were 7.66% and 6.42% lower than that of Fe under optimal conditions (dosage = 9.38 g/L, pH = 8.94, settling time = 52.9 min). The present study established that Fe recovered could be exploited as an eco-friendly coagulant to replace FeCl3 in the landfill leachate treatment.

A review of iron use and recycling in municipal wastewater treatment plants and a novel applicable integrated process

Chemical methods are expected to play an increasingly important role in carbon-neutral municipal wastewater treatment plants. This paper briefly summarises the enhancement effects of using iron salts in wastewater and sludge treatment processes. The costs and environmental concerns associated with the widespread use of iron salts have also been highlighted. Fortunately, the iron recovery from iron-rich sludge provides an opportunity to solve these problems. Existing iron recovery methods, including direct acidification and thermal treatment, are summarised and show that acidification treatment of FeS digestate from the anaerobic digestion-sulfate reduction process can increase the iron and sulphur recycling efficiency. Therefore, a novel applicable integrated process based on iron use and recycling is proposed, and it reduces the iron salts dosage to 4.2 mg/L and sludge amount by 80%. Current experimental research and economic analysis of iron recycling show that this process has broad application prospects in resource recovery and sludge reduction.

A comprehensive review on the coagulant recovery and reuse from drinking water treatment sludge

The main treatment unit in conventional systems for surface water is coagulation-flocculation (CF) process, which consumes huge quantities of coagulant, and produces large volume of sludge. The produced sludge is known as one of the components of water treatment sludge (WTS), which is considered as a global issue and hot topic require careful attention from the plant operators and sludge managers to be managed sustainably with applying an ecofriendly method. Among the suggested technologies, recovery and reuse of coagulants from WTS show the potential to decrease the waste disposal and chemicals usage for drinking water treatment significantly. So, this comprehensive review provides a useful insight into environmental and health problems of WTS, reports the sources, physicochemical properties of sludge, describes different sludge management methods by more focus on coagulant recovery (CR), which significantly point out the different aspects of WTS recovery and reuse, and eventually, economic evaluation of the CR process was also discussed. The results of this review confirm that coagulants can be recovered from WTS by different methods and also will be reused for multiple times in the removal of pollutants from water and wastewater. Moreover, the recovered coagulants can be used as building and construction materials, constructed wetlands substrate and other aims.

Chemically enhanced primary treatment of municipal wastewater with ferrate(VI)

Chemically enhanced primary treatment (CEPT) with ferrate(VI), a multifunctional treatment agent, was investigated for the treatment of municipal wastewater in a laboratory-scale study. The treatment performance was evaluated at different ferrate(VI) doses (0.0-9.0 mg/L as Fe) and pH (6.0 and 7.5). The optimal removals of total suspended solids (TSS) (52%), total chemical oxygen demand (COD) (34%), and total phosphorus (47%) were achieved at the highest ferrate(VI) dose (9.0 mg/L as Fe) and the weakly alkaline condition (pH 7.5). The pollutant abatements principally ascribed to the formation of large-sized aggregate and ensuing sedimentation fell within the reported ranges of CEPT with traditional coagulants. However, different from conventional CEPT, ferrate(VI) appreciably removed recalcitrant dissolved organic phosphorus (49%) and simultaneously inactivated total coliform (3.30 log removal) and Escherichia coli (3.67 log removal) at 9.0 mg/L Fe(VI) and pH 7.5. The CEPT with ferrate(VI) offers an innovative alternative for improving municipal wastewater treatment. PRACTITIONER POINTS: Ferrate(VI) represents a promising agent for chemically enhanced primary treatment (CEPT) of municipal wastewater. CEPT with ferrate(VI) can effectively alleviate TSS, total COD, and total P via the formation of large-sized aggregates and ensuing sedimentation. Ferrate(VI) can substantially remove recalcitrant dissolved organic phosphorus in municipal wastewater. Different from other CEPT coagulants, ferrate(VI) can appreciably inactivate bacterial indicators during CEPT. Higher ferrate(VI) dose and weakly alkaline pH favor the performance of ferrate(VI) CEPT.

Revisiting Chemically Enhanced Primary Treatment of Wastewater: A Review

Chemically enhanced primary treatment (CEPT) is a process that uses coagulant and/or flocculant chemicals to remove suspended solids, organic carbon, and nutrients from wastewater. Although it is not a new technology, it has received much attention in recent years due to its increased treatment capacity and related benefits compared to the conventional primary treatment process. CEPT involves both physical and chemical processes. Alum and iron salts are the commonly used coagulants in CEPT. Several types of anionic, cationic, and uncharged polymers are used as flocculants, where poly aluminum chloride (PACL) and polyacrylamide (PAM) are the widely used ones. Some of the coagulants and flocculants used may have inhibitory and/or toxicity effects on downstream treatment and recovery processes. There has been an increasing amount of work on the treatment of wastewaters from various sources using CEPT. These wastewaters can range from municipal/domestic wastewater, combined sewer overflow, landfill leachate, cattle manure digestate to wastewaters from textile industry, pulp and paper mill, slaughterhouse, milk processing plant, tannery and others. In recent cases, CEPT is employed to enhance carbon redirection for recovery and substantially reduce the organic load to secondary treatment processes. CEPTs can remove between 43.1–95.6% of COD, 70.0–99.5% suspended solids, and 40.0–99.3% of phosphate depending on the characteristics of wastewater treated and type of coagulants and/or flocculants used. This article reviews the application, chemicals used so far, removal efficiencies, challenges, and environmental impacts of CEPT.

Material inter-recycling for advanced nitrogen and residual COD removal from bio-treated coking wastewater through autotrophic denitrification

For wastewaters containing high strength sulfide and nitrogen (e.g. coking wastewater), sulfide might be precipitated and recovered using ferrous salt. This study systematically investigated the feasibility of recovered and precipitated FeS (comparing to commercial FeS minerals) to support autotrophic denitrification for advance nitrogen removal from bio-treated coking wastewater in fluidized bed reactors. The reactor with precipitated FeS could achieve simultaneous removal of NO₃^--N and inert COD with high efficiencies of around 96.3% and 30.5%, at NO₃^--N and COD loading rates of 4.18 mg·L^-1·h^-1 and 8.06 mg·L^-1·h^-1, respectively. Whereas, the performance of commercial FeS reduced gradually and irreversibly after two days, which became completely ineffective after 40 days. Thiobacillus and Rhodanobacter dominated the biomass, which played a key role in the FeS-based denitrification process. This material inter-recycling concept benefits an advance and more sustainable treatment of wastewaters with high strength sulfide and nitrogen.

Optimization of aluminium recovery from water treatment sludge using Response Surface Methodology

For decades, water treatment plants in Malaysia have widely employed aluminium-based coagulant for the removal of colloidal particles in surface water. This generates huge amount of by-product, known as sludge that is either reused for land applications or disposed to landfills. As sludge contains high concentration of aluminium, both can pose severe environmental issues. Therefore, this study explored the potential to recover aluminium from water treatment sludge using acid leaching process. The evaluation of aluminium recovery efficiency was conducted in two phases. The first phase used the one factor at a time (OFAT) approach to study the effects of acid concentration, solid to liquid ratio, temperature and heating time. Meanwhile, second phase emphasized on the optimization of aluminium recovery using Response Surface Methodology (RSM). OFAT results indicated that aluminium recovery increased with the rising temperature and heating time. Acid concentration and solid to liquid ratio, however, showed an initial increment followed by reduction of recovery with increasing concentration and ratio. Due to the solidification of sludge when acid concentration exceeded 4 M, this variable was fixed in the optimization study. RSM predicted that aluminium recovery can achieve 70.3% at optimal values of 4 M, 20.9%, 90 °C and 4.4 h of acid concentration, solid to liquid ratio, temperature and heating time, respectively. Experimental validation demonstrated a recovery of 68.8 ± 0.3%. The small discrepancy of 2.2 ± 0.4% between predicted and validated recovery suggests that RSM was a suitable tool in optimizing aluminium recovery conditions for water treatment sludge.

A novel low cost microalgal harvesting technique with coagulant recovery and recycling

In this study, a novel low cost and sustainable microalgal harvesting technique was developed using the concept of coagulant recovery concentration and recycling. Al³⁺ can be recovered from harvested Scenedesmus acuminatus biomass with 0.1 M HCl, at an acid solution-biomass ratio of 250 ml g^-1. The residual Al³⁺ content in the purified biomass was reduced to 0.11 ± 0.0006 mg g^-1, while a higher content of 59.74 ± 3.11 mg g^-1 was found in the coagulation harvested biomass. The recovered Al³⁺ solution was concentrated 25 times and then reused for the harvesting of S. acuminatus. The Al³⁺ recovery and reuse were repeated 5 times, and the harvesting efficiencies were found higher than the fresh Al³⁺ as a result of the presence of extracellular polymeric substances in the recovered coagulant solution which aided the coagulation process. According to the technical-economic analysis, the cost of chemicals decreased 50% after 5 times recycling.

The feasibility study of autotrophic denitrification with iron sludge produced for sulfide control

Ferric iron is widely dosed in wastewater treatment plants dealing with sulfide for septicity control, which generates a great amount of iron-rich chemical sludge that is challenging and costly to dispose. This study investigates the feasibility of using this iron sludge as the electron donor for autotrophic denitrification, not only realizing high nitrogen removal efficiency without additional carbon source requirement, but also partially mitigating iron-rich chemical sludge disposal and reduce sludge production by enriching low-yield autotrophic denitrifiers in the system. Both batch tests and performance monitoring of a lab-scale up-flow anaerobic sludge blanket reactor with a more than 300 days of operation were conducted. All the results confirmed the feasibility of using iron sludge as electron donor for autotrophic denitrification. The nitrate reduction rate with iron sludge was highly influenced by the type of ferrous electron donor and the electron donor/acceptor ratio. Ferrous hydroxide had significantly higher nitrate reduction rate than ferrous sulfide at the same electron donor/acceptor ratio. The nitrate reduction rate also accelerated with the increase of the electron donor/acceptor ratio. However, if the total surface area of the iron sludge is considered for comparison, it was shown that ferrous hydroxide and ferrous sulfide provided similar nitrate reduction rates of around 0.02 mmol N/m²/d in this study, indicating total surface area would be the key parameter for denitrification efficiency for the solid phase electron donor.