containerized water treatment plant for sewage treatment
Although there are many advanced water treatment technologies available, the technologies implemented in low-cost POU treatment systems are mainly characterized as mature and fundamental, aiming to remove waterborne pathogens. The available technologies are mainly flocculation and coagulation, filtration, and disinfection. Flocculation and coagulation remove the turbidity in the water, which reduces the supporting structure of microorganisms. This resulted in the removal of microorganisms.13 Filtration removes microorganisms by size exclusion, whereby microorganisms larger than the pore size of the filter will be retained within the system.11 Disinfection is the inactivation and destruction of microorganisms in the system to a safe level.14 Photographs of a flocculant and a membrane system are shown in Fig. 1.
Fig. 1Photographs of (left) bioflocculant and (right) WateROAM’s portable membrane system (reproduced with permission from WateROAM)
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Coagulation and flocculation are low energy and reliable water treatment processes. Coagulation is the destabilization of smaller particles (0.01–1 µm), resulting in the formation of larger particles. Flocculation is the formation of flocs from the destabilized particles.15 The flocculated particles can then be removed via settling or filtration.
Coagulation is able to effectively remove turbidity, colloids,16 and protozoan cysts.11 Coagulation, however, is unable to achieve a stable coliform removal.17 It has to be coupled with additional treatment process to improve the overall pathogen removal efficiency. When coagulation is used as a pretreatment or together with a membrane process, it can improve the removal of Cryptosporidium,18 E. coli and MS2.19 In addition, coagulation will improve the disinfection process by removing turbidity and reduce the scavenging of disinfectant by natural organic matters.20
The effective use of conventional coagulants such as alum, iron salt, and lime requires proper training and technical skills. Therefore, in the past, it had been mostly deployed for communal use in rural regions. Direct household usage of coagulant is rare.16,21 However, in the past decade, optimization of coagulant dosage, development of natural coagulants, and addition of disinfectant have allowed manufacturers, such as Procter & Gamble Co. (P&G) and Poly Glu International Co. to produce coagulants/flocculants that are stored in small POU sachets. This makes it easier for usage and distribution.
The PUR sachet from P&G is a combined flocculation/disinfection system. Ferric sulfate flocculates the destabilized particles within the water, while the calcium hypochlorite disinfects the water.22 In a study in western Kenya, sachets from P&G of different dosages were used to treat turbid water of various sources. In the study, high dose formulation of the flocculant-disinfectant product achieved water quality with mean E. coli concentration of 0 mg/L. The low dose formula, when used in high turbid water, was unable to consistently achieve drinking water quality.17 The use of P&G’s flocculant-disinfectant sachet in rural Guatemala reduced the episode of diarrhea per person by 23.8%.23 In a case study by P&G in Vietnam, the PUR sachets achieved >5 log virus removal. 4 log removal and 3.6 log removal were also achieved for Cryptosporidium and Giardia, respectively.22 The PUR sachet was also shown to be able to achieve >99% arsenic removal and >8.2 log removal of E. coli.21
Polyglutamic acid (PGA) is a natural, biodegradable, and edible bio-flocculant that is used at industrial scale24 and is the main ingredient in the Poly Glu sachet. It is produced from Bacillus licheniformis and Bacillus subtilis.25 PGA has high flocculating activity and high yield, resulting in the production of a smaller volume of sludge. It has better performance than aluminum sulfate in turbidity removal.26 Unlike the inorganic coagulant/flocculant, suspended solids removal by PGA is dose-dependent and is independent of temperature and pH, making it an ideal option for treating a wide range of water.27,28,29,30 PGA is also capable of adsorbing copper ions, removing harmful copper ions from the water source.31 In addition, in a patent submitted by Masamichi Mutou, a mixture containing γ-PGA with cross-link product of γ-PGA, sodium, calcium, and aluminum was able to inactivate microorganism.32 The components mentioned in the patent are used in PGα21, an ingredient used in PolyGlu; hence reducing the need for additional disinfectant within the PolyGlu sachet. However, the lack of residual chlorine may result in recontamination.
Coagulation and flocculation have shown success in treating drinking water and are mainly used for high turbid water. However, coagulation and flocculation cannot be a standalone water treatment solution. In the PUR sachet, disinfectants were added for effective removal of the pathogen. PolyGlu, on the other hand, requires proper water storage to prevent recontamination. While the PUR sachet and PolyGlu sachet had shown positive results in the removal of arsenic and copper, more studies are required to evaluate the effectiveness in removal of other potential heavy metals that can potentially reside in water sources. In addition, POU coagulant and flocculant are for single-use. This makes the process expensive in the long run. Ferric sulfate, a common coagulant, has shown potential in recovery and reuse.33 It can be explored to lower the operating expense (OPEX) of drinking water treatment in developing communities. Electrocoagulation, a process that generates aluminum hydroxide coagulant, has also shown potential in decentralized water treatment. Wind-powered electrocoagulation system was capable of removing up to 90% of microalgae and 97% of dissolved dye from synthetic wastewater within 72 h.34 This process demonstrated its ability to remove both pathogen and dissolved chemicals.
Filtration is a simple water treatment process capable of removing colloids, suspended solids, and pathogens from drinking water sources. Its main removal mechanism is size exclusion. A well-designed filtration system will be able to generate a clean stream of drinking water. The size of different constituents commonly found in water sources and the pore size of conventional filtration process are summarized in Fig. 2. Filtration technologies implemented in POU are mainly biosand filtration (BSF) and membrane filtration.
Fig. 2Size of water constituents and pore size of various filtration technologies (adapted with permission from ref.52 copyright Elsevier 2011)
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In membrane filtration, external driving force is typically required to achieve the desired flow rate, and the external driving force required shall depend on the membrane pore size, surface area, and influent water quality. Compared to a membrane with larger surface area and pore size, higher applied pressure is required to filter water across a smaller membrane area with smaller pore size. In addition, maintenance is required in the filtration process. After prolong period of filtering, a foulant layer will be deposited on the filter layer. In sand filtration, flux can be recovered by backwash. In membrane system, backwash removes the reversible fouling, while chemical cleaning is required to remove biofouling and scaling.
BSF is a promising POU treatment process with more than 500,000 people using it worldwide. It can be easily constructed using raw materials that are locally sourced. The concept of BSF is similar to that of a conventional slow sand filter. However, BSF experiences varying flowrate and intermittent filtration through the sand layer. The outlet of the POU is located higher than the sand layer, allowing the sand layer to be saturated with water throughout the operation. This allows for microbial growth, developing biofilm around the filter media and sand particles. Excessive biofilm growth in the biolayer forms the Schmutzdecke, which removes larger microorganisms, colloids, and contaminants from the water sources.35,36
BSF is capable of reducing turbidity and removing E. coli. While a new BSF POU filter only achieved 63% E. coli removal, 98% removal was observed in a mature filter.35 However, the treated water does not meet the WHO drinking water guideline. By replacing the sand with iron-coated sand, more than 2 log removals of E. coli was observed when it was fed with water with > 103 CFU/mL E. coli concentration.37 Increasing the flowrate, however, reduced the retention time and overall effectiveness of the filter.38 This indicated that the sizing of the BSF is crucial in order to accommodate the target loading water volume. Removal of coliform will not be effective if the system is under-designed. Virus removal was also observed in BSF, achieving 1.3 log removals of echovirus 12 with fresh BSF and 2.2 log removal after maturation.38 Filter depth was found to affect the removal of virus. A BSF with 54.3 cm filter depth achieved 6.7 log reduction of MS2, while a BSF with 5.4 cm filter depth achieved 3.4 log reduction of MS2.36 In a separate study, BSF with 10% zero-valent iron achieved more than 5 log MS2 and rotavirus removal.39 This indicated that virus removal was mainly due to sorption to the filter media. Anaerobic BSF, with addition of ferrous iron, was also found to be able to reduce arsenic contamination in water.40 These studies showed that the filter media would affect the type of contaminant removed by BSF and the potential to remove other contaminants.
BSF has shown success in reducing diarrhea episode globally. It has also been shown to have a long filter life, with a BSF in Cambodia lasting up to 8 years of continuous usage.41 BSFs have been deployed in real-world condition to treat water sources. The performance of BSFs deployment in various countries is summarized in Table 1.
Table 1 Case studies of BSF deploymentsFull size table
Membrane filtration is a mature technology and is one of the most effective drinking water treatment processes. Clasen et al.42 concluded that membrane filtration is the most effective means of preventing diarrhea. It provides an absolute barrier for microorganisms, retaining them within the water source. However, membrane filters require an external driving force such as electrical pumping. In the past decade, innovative designs and optimization allowed manufacturers to produce off-grid or non-electric driven membrane filter systems. Companies such as Wateroam, Icon lifesaver, and Villagepump integrated a manual pump within the membrane system, making the system grid-independent. Grid-independent water filtration system is crucial because many places around the world do not have continuous electricity supply. In South Asia, more than 50% of people living in rural areas do not have electricity.43 Therefore, grid-independent water filtration system is highly recommended in rural areas. A list of membrane filtration POU designed for developing communities deployed in Asia is tabulated in Table 2.
Table 2 Membrane filtration POU deployment in AsiaFull size table
Though some membrane filtration systems have high initial cost, their flowrate and lifespan resulted in lower cost per litre of water produced. ROAMplus, a product from WateROAM, has an estimated lifespan of 2 years and a flowrate of 250 L/h. This resulted in an estimated cost of <USD 0.001 per litre, assuming the system is used 5 h per day, 350 days per year. In addition, comparing with BSF, membrane filter systems have significant higher flow rate. The high membrane packing density allows for high membrane surface area to volume ratio, resulting in higher flow rate. In addition, membrane filtration is capable of removing turbidity and pathogens. As seen from Table 2, ultrafiltration (UF) membrane systems achieve more than 6 log bacteria removal,44,45,46 while microfiltration (MF) membrane systems achieve at least 4 log bacteria removal.47 Table 2 also shows that UF is capable of achieving more than 3 log virus removal. This high microorganism removal makes membrane systems more reliable and effective than conventional ceramic candle filters.48 Long-term studies revealed that membrane filtration systems, with proper maintenance, could consistently produce similar water quality results48,49 over extended periods of time.
Though the idea of household slow sand filtration existed since the 1980s, extensive researches were only conducted since 2000. BSF demonstrated great potential for water treatment. Changes in operating conditions and filter media compositions may yield different results. One solution is to add biochar into BSF systems to improve pathogen removal. Similar to BSF, biochar is a low-cost and sustainable adsorbent that can be made from locally sourced materials. Through pyrolysis or degasification, organic materials can be converted into biochar.50 While it is primarily used for adsorption, it was found to be able to inactivate microorganisms.51 The addition of biochar into BSF may improve pathogen, organic contaminant, and heavy metal removals. However, the capability of pathogen removal is dependent on the biochar characteristics and operating condition. While BSF is able to reduce pathogens in the water, the absence of residual disinfectants could allow recontamination to occur. In addition, disinfection process is required downstream to ensure the treated water is capable of meeting the WHO drinking water guidelines.
While both BSF and membrane POU systems are able to remove pathogens effectively, they are unable to remove dissolved solids. The pore sizes of the MF or UF membranes are too large to retain dissolved solids, allowing the dissolved solids to permeate through.52 Dissolved solids such as heavy metals can be removed by dense membrane systems such as the reverse osmosis (RO) process, which is a commonly used dense membrane to remove dissolved solids from water sources in desalination.53 However, due to the complexity of RO system, requirement for stable electricity supply, and maintenance, RO systems are not commonly deployed in rural areas. In addition, the use of RO also increases the operation cost of the water treatment system. Another dense membrane process, forward osmosis, is a favorable technology in treating chemically contaminated wastewater. Products from Hydration Technology Innovations had developed POU systems that utilized forward osmosis to filter water source. The draw solution, however, is not reusable and the system has a relatively low water flux.
Unfavorable water condition may result in rapid fouling of membrane, increasing maintenance frequency, and reducing water production. Water with elevated concentration of dissolved organic matters (DOM) will result in organic fouling. The formation of biofilm on membrane surface results in biofouling. Lastly, accumulation of particles on membrane surface as well as within the membrane pores results in particle fouling. These fouling will reduce permeate flux and water filtration rate. Chemical cleaning and backwash will be required once the flux is below acceptable limits.54 In cases where water conditions are unfavorable, pretreatment is required prior to membrane process.