Abstract
Currently, reservoirs, lakes, rivers etc. are being overloaded by the demand for fresh water, due to rapid industrialization and population explosion, and also the effluents from industries and domestic wastewater are continuously polluting these resources. To address this issue, several decentralized wastewater treatment system (DWTS) have been installed all over the globe to reuse and recycle wastewater/graywater for non-potable uses such as fire protection, toilet-flushing, and landscape irrigation. In this review, a comparison between different DWTS was carried out to evaluate their performance, merits and limitations. Hybrid technologies like the electrically enhanced biomass concentrator reactor and integration of physical/ biological methods with bio-electrochemical systems such as microbial fuel cells were found to be the most promising methods for near complete removal of pollutants from wastewater and also the issue of membrane fouling was reduced to a good extent.
INTRODUCTION
According to Global Health Observatory data, around 71% the world population of 5.2 billion in 2015 used a safe and securely supervised drinking water supply in almost 96 countries, including 1.9 billion people from rural areas (WHO & UNICEF 2015). By the end of the year 2015, almost 181 countries had effectively attained more than 75% coverage of at least fundamental drinking water services, but according to WHO (2017), around 844 million people still needed a basic supply of drinking water and approximately 159 million people were using drinking water straight from surface water resources.
Only 2.9 billion people (approx. 39%) had a proper sanitation facility in almost 84 countries and 1.9 billion people (approx. 27%) utilized private sanitation services that are connected directly to sewers for further treatment of wastewater (WHO & UNICEF 2015). But around 2.3 billion people still need a proper sanitation facility and nearly 892 million people did not have a proper sanitation facility and they still practiced open defecation.
To overcome this issue, notable development has been made in water and wastewater treatment for urban areas, but this is insufficient to cope with today's requirements. The prime investments on conventional wastewater treatment units or plants are their high maintenance and capital cost and large area requirement. The reason for inadequate operation of these treatment plants include lack of local expertise, confined local budgets and insufficient funds in most of the developing countries (van Afferden et al. 2015; Chirisa et al. 2017; Zaharia 2017). To solve this problem decentralized wastewater treatment systems (DWTS) as a government initiative are becoming popular among society to reduce the issue of improper sanitation and also to provide optimum water supply for non-potable purposes (Bieker et al. 2010; Larsen & Maurer 2011).
Over 5,000 years, water and wastewater reuse has been implemented globally; however, over the last 100 years attempts have been made by several countries to improve and produce high quality reused water for non-potable and irrigation purposes (Amerasinghe et al. 2013). Domestic wastewater (DWW) is composed of 70% organic content and 30% inorganic content. The treatment systems mainly depend on this 70% organic part of wastewater. This organic matter consists of a large group of fibers (20.64%), followed by proteins (12.38%) and sugars (10.65%) (Huang et al. 2010). DWW is further separated into black water and graywater (Gross et al. 2015). Blackwater comprises urine and fecal matter (40%) and also contains human waste, food scraps, paper and detergent residues, and graywater is a type of DWW which originates from showers and sinks (15%), washing machines (10%) and bathrooms (30%) (Albalawneh & Chang 2015; Rose et al. 2015).
The reclaimed water from DWTS is reused globally for landscape and agricultural irrigation, industrial use, non-potable domestic uses, groundwater recharge and also for recreational uses (Lautze et al. 2014; González et al. 2015). Figure 1 shows global water reuse after treatment.
This study provides a wide overview of DWTS, existing technologies and their applications for domestic wastewater and compares the strengths and weaknesses of various treatment options including some emerging technologies in decentralized treatment. The main aim is to provide a context-sensitive and comparative study of the performance aspect of technologies that are used for domestic wastewater treatment.
METHODOLOGY
The author has used some keywords to look for recent (papers published in previous decade) or especially pertinent published papers with respect to the title of this review paper. Some of them are: decentralized wastewater treatment, graywater, electro-biochemical, MFCs, membrane bioreactor, MBR, MBBR, BCR, reclaimed wastewater, reuse, and membrane fouling. SCI, Scopus and Google Scholar were used as sources for survey and review of the literature. The papers relevant to this area were collected and thoroughly analyzed for this review.
DECENTRALIZED WASTEWATER TREATMENT SYSTEMS
The limitations like land availability, complexity and cost-effectiveness of centralized wastewater treatment systems increase the urge for essential innovation in design of economic and environmentally sustainable wastewater treatment system such as DWTS (Sathe & Munavalli 2019). From the economic point of view, DWTS are more advantageous over centralized wastewater treatment systems as suggested by various researchers (Libralato et al. 2012; Starkl et al. 2012; Singh et al. 2015).
Basically two major objectives for wastewater treatment and management have been established: firstly, to promote and protect human health from various diseases and, secondly, to provide ecosystem protection and desirable water quality by shrinking the effects of immoderate pollutants that are released into the environment (Capodaglio et al. 2017). These objectives can be fulfilled by implementing either of the treatment systems (decentralized or centralized). In centralized systems huge volumes of wastewater are treated after being transported in large pipes, which requires many excavations and bigger diameter of manholes (Arora et al. 2016). Centralized wastewater treatment systems constitute a single treatment scheme and are inappropriate for the reuse of water as it is difficult to effectively manage a large volume (Vol) of recovered water (Arias et al. 2019; de Souza Celente et al. 2019). Instead DWW can also be treated on-site with fewer excavations where wastewater is produced directly from a colony or a single household or a thinly populated, rural and remote area which generally includes reusing the effluent (Nasr & Mikhaeil 2013; Chirisa et al. 2017).
DWTS for recycling of DWW are reliable as the treated wastewater effluent was up to the desired quality and was reused within the same locality or community (Nivala et al. 2019; Philip et al. 2019). A decentralized system is a system which involves collection, treatment, recycle/reuse and disposal of wastewater generated from kitchens, toilets or bathrooms, close to the point of generation, which makes this system more sustainable and an appropriate fit (Zaharia 2017). These systems are planned to operate for low population density area.
DWTS generally consist of two units, one primary and the other secondary treatment. Primary treatment involves the settlement of heavy solids and flocs at the bottom of the tank while other substances like oil and grease are removed from the surface. After the settled particles and the floating materials are removed from the tank, the desired wastewater is said to be partially treated. Secondary treatment is done to remove dissolved and suspended solids to obtain better quality effluent for the wastewater for non-potable uses (Arora et al. 2016). The major approach of DWTS is to minimize the utilization of freshwater resources, reduce the amount of pollutants crossing the boundary and finally maximize the reuse of water.
The concept of decentralized sanitation and reuse notably improves the wastewater sanitation problem in small urbanized rural settlements in regions of Latin America by reducing the final cost of the treatment as a consequence of reclamation of water and sewage sludge waste reuse for agricultural purposes (Cardona et al. 2019). The government of Indonesia has successfully planned and implemented DWTS for improving public health and quality of the environment in urban areas (Yulistyorini et al. 2019).
DWTS can efficiently and effectively treat DWW to maintain water quality and public health, and can support local supply of water, as the treated wastewater of this system will remain in the local watershed. By installing this system, it may become easier for a local community to establish water reuse systems for non-potable purposes, and hence it may reduce unsuitable demand for fresh and treated drinking water (Capodaglio et al. 2017).
The simplest and oldest form of DWTS is a conventional septic tank, which is also called a cesspool, typically installed in isolated locations in developing countries, and treating the wastewater by settlement of suspended solids and also achieving some anaerobic digestion (Moussavi et al. 2010). This system works effectively in hotter climate and 50% of organic load can be removed but it is less effective in removing pathogens, because of which it requires further treatment thereby increasing the complexity and cost of the system. In some part of Eastern EU countries 70% of the wastewater processed by this technique is still under use for water supply (Istenic et al. 2015).
Another common, simple and old form of DWTS is WSPs (waste stabilization ponds) that include purely aerobic maturation ponds, facultative ponds and anaerobic ponds. WSPs have simplicity in design, have long retention times, are low-cost and utilize negligible amount of energy. These ponds can also provide other economic benefits, as these ponds could provide a better surrounding for the growth of fish like tilapia, and also the ponds possess high algae concentrations, which is very good for irrigation (Capodaglio et al. 2017). But the major limitation of WSPs is that they require relatively large land areas for the wastewater treatment (US EPA 2015).
Generally, most of the DWTS opt for gravity flow rather than pumping, which results in reduction of the cost and energy demand for an appropriate size of the system so that it can fit into small communities. New and advanced technologies of DWTS can result in better treatment levels as compared to a conventional centralized treatment system and can meet desired treatment goals keeping in mind the site conditions and local environmental protection.
RECLAIMED GRAYWATER REUSE STANDARD AND GUIDELINES WORLDWIDE
Water discharged from domestic sources like from washing machines, hand basins and kitchen sinks but excluding water from bidets, urinals and toilets is termed as graywater (Gross et al. 2015). Graywater usually contains various macro-pollutants, but its properties get altered depending upon the habits and use of residents living in particular households and also varies in numerous other countries.
The water shortage is growing day by day throughout the globe, and to reduce the scarcity of water, many countries have adopted the technique of non-potable reuse of the wastewater for toilet flushing, green irrigation, cleaning of pavement, etc. (Jamwal & Mittal 2010).
To reuse the reclaimed graywater, it has to fulfill four major criteria, i.e., environmental tolerance, hygienic safety, economic and aesthetics achievability (Gildemeister et al. 2005; Gross et al. 2015). One should remember that the required quality of reclaimed water is totally dependent upon its end use, which further decides the degree of treatment by different technologies, varying from very simpler to the complex and advanced ones.
For the protection of human health, various standards and guidelines for reclaimed water reuse were set up by several countries as mentioned in Table 1.
Worldwide guidelines for reuse of water for non-potable purposes (Cecconet et al. 2019a)
| Institution/Country | BOD (mg/L) | TSS (mg/L) | Turbidity (NTU) | pH | Residual chlorine (mg/L) | Microorganisms (CFU 100/mL) | Applications |
|---|---|---|---|---|---|---|---|
| WHOa | ≤10 | ≤10 | – | – | – | Thermo-resistant coliforms: ≤10 | Toilet flushing |
| USAb | ≤10 | – | ≤2 (avg.) <5 (max) | 6–9 | ≥1 | Fecal coliforms (FC): undetectable Escherichia coli: <100 Total coliforms (TC): <2.2(avg) <23 (max) | Unrestricted urban reuse/ toilet flushing |
| UKc | – | 10 | <10 | 5–9.5 | <2 | E. coli: <25 FC: 1,000 | Toilet flushing |
| Australiad | <10 | <10 | <2 (95%) <5 (max) | 6.5–8.5 | – | E. coli< 1 | Toilet flushing/washing machine |
| Canadae | ≤20 | ≤20 | ≤5 | – | ≥0.5 | E. coli: ≤200 Thermo-resistant coliforms: ≤200 | Toilet and urinal flushing |
| Germanyf | <5 | – | – | – | – | TC count: <100 FC: <10 Pseudomonas aeruginosa: <1 | Service water |
| Japanf,g | – ≤20 | – | ≤2 Not unpleasant | 5.8–8.6 5.8–8.6 | Retained ≥0.4 | E. coli: undetectable TC count: <50 | Toilet flushing/landscape irrigation |
| Italyh | ≤20 | ≤10 | – | 6.0–9.5 | – | E. coli: ≤10 | General |
| S. Koreai | <10 | – | <10 | 5.8–8.5 | >0.2 mL/L | E. coli: undetectable | Toilet flushing |
| Israelj | <10 | <10 | <20 (mean < 10) | – | – | FC: <1 E. coli: < 400 (mean <100) | Toilet flushing |
| Chinak | <10 <20 <6 | – | <5 <20 <5 | 6–9 6–9 6–9 | >1 (after 30 min), >0.2 (at point of use) >1 (after 30 min), >0.2 (at point of use) >1 (after 30 min), >0.2 (at point of use) | FC: <3 FC: <3 FC: <3 | Toilet flushing Irrigation Washing |
| Institution/Country | BOD (mg/L) | TSS (mg/L) | Turbidity (NTU) | pH | Residual chlorine (mg/L) | Microorganisms (CFU 100/mL) | Applications |
|---|---|---|---|---|---|---|---|
| WHOa | ≤10 | ≤10 | – | – | – | Thermo-resistant coliforms: ≤10 | Toilet flushing |
| USAb | ≤10 | – | ≤2 (avg.) <5 (max) | 6–9 | ≥1 | Fecal coliforms (FC): undetectable Escherichia coli: <100 Total coliforms (TC): <2.2(avg) <23 (max) | Unrestricted urban reuse/ toilet flushing |
| UKc | – | 10 | <10 | 5–9.5 | <2 | E. coli: <25 FC: 1,000 | Toilet flushing |
| Australiad | <10 | <10 | <2 (95%) <5 (max) | 6.5–8.5 | – | E. coli< 1 | Toilet flushing/washing machine |
| Canadae | ≤20 | ≤20 | ≤5 | – | ≥0.5 | E. coli: ≤200 Thermo-resistant coliforms: ≤200 | Toilet and urinal flushing |
| Germanyf | <5 | – | – | – | – | TC count: <100 FC: <10 Pseudomonas aeruginosa: <1 | Service water |
| Japanf,g | – ≤20 | – | ≤2 Not unpleasant | 5.8–8.6 5.8–8.6 | Retained ≥0.4 | E. coli: undetectable TC count: <50 | Toilet flushing/landscape irrigation |
| Italyh | ≤20 | ≤10 | – | 6.0–9.5 | – | E. coli: ≤10 | General |
| S. Koreai | <10 | – | <10 | 5.8–8.5 | >0.2 mL/L | E. coli: undetectable | Toilet flushing |
| Israelj | <10 | <10 | <20 (mean < 10) | – | – | FC: <1 E. coli: < 400 (mean <100) | Toilet flushing |
| Chinak | <10 <20 <6 | – | <5 <20 <5 | 6–9 6–9 6–9 | >1 (after 30 min), >0.2 (at point of use) >1 (after 30 min), >0.2 (at point of use) >1 (after 30 min), >0.2 (at point of use) | FC: <3 FC: <3 FC: <3 | Toilet flushing Irrigation Washing |
BOD, biochemical oxygen demand; TSS, total suspended solids.
cBritish Standards Institution: BS-8525-2 (2011); do Couto et al. (2015).
jPidou et al. (2007); Oh et al., (2018); SI-6147 as reported in Oron et al. (2014).
TECHNOLOGICAL ADVANCEMENT IN DWTS
The focus of this review paper was to summarize the technological improvement, advancement and performance of the different techniques used for DWTS. The technical advancement over the decade in DWTS is shown by three tables, viz. Tables 2–4, and is discussed in later sections.
Performance inventory of biological and physical techniques reported in the literature
| Country | Application (scale) | Sampling location/type | Treatment scheme | SRT (days) | HRT (hours) | Removal (%) | Reference | |||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| BOD | COD | TN | TSS | TC (CFU/100 mL) | NH3-N | TP | FC | |||||||
| Germany | Non-potable (laboratory-scale/ BUSSE reactor) | Synthetic | SM-SBR | >250 | 13–60/33–100 | – | 79/89 | 37/41 | – | – | 99/98 | 50/71 | – | Gildemeister et al. (2005) |
| China | – | University | CABR | – | 48 | – | 79 | 19.22 ± 5.63 | 81.92 ± 3.53 | 77.35 | – | 30.86 ± 4.68 | 99.82 | Feng et al. (2008) |
| China | Toilet flushing (laboratory-scale) | Bathing wastewater | SMBR | 90 | 4 | 93.33 ± 1.03 | 87.57 ± 2.73 | 57.74 | 99.98 | – | 98.78 ± 0.21 | 90 | – | Jifeng et al. (2008) |
| Egypt | (Laboratory-scale) | Wastewater | USBR | >300 | 20 | 81 | 84 | – | 89 | – | _ | – | – | Sabry (2010) |
| Iran | (Pilot-scale) | Residential wastewater | UST | >350 | 24 | 85 | 77 | – | 86 | – | – | – | – | Moussavi et al. (2010) |
| Iran | Irrigation (Pilot-scale) | Synthetic sewage | ASR | >300 | 24 | 79.4 | 86.2 | – | 95 | – | – | – | – | Jorsaraei et al. (2014) |
| USA | (Laboratory-scale) | Synthetic wastewater | BCR 1. conventional 2. hybrid | 6 15 | 9 9 | – | 93 97 | 43–53 75–79 | – | – | – | – | – | Scott et al. (2013) |
| Spain | Toilet flushing (prototype) | Showers and bathrooms | MBR | – | 19.5 | 95 | 90 | – | 98 | – | – | – | – | Santasmasas et al. (2013) |
| London, UK | Non-potable (Pilot-scale) | Composite wastewater sample | Septic tank 1. conventional 2. baffled | – | 72 72 | 68.4 76.5 | 65.3 74 | 26.8 31.2 | 65.3 76 | – | – | 29.3 33.1 | 86 95 | Nasr & Mikhaeil (2013) |
| Iran | (Laboratory-scale) | STP | Bio-cache | 45 | 2 | 88 | 78 | – | 72 | 95 | 75 | 40 | 93 | Valipour et al. (2014) |
| Malaysia | Non-potable | Household | Aerobic digestion unit + H2O2 | – | 5 | – | 68 | – | 88 | 4 × 106 | – | – | – | Teh et al. (2015) |
| Jordan | Non-potable (laboratory-scale) | Graywater | SMBR | 42 | 7 | – | 88 | – | – | 29 | 8.98 | 56 | 26 | Bani-Melhem et al. (2015) |
| Italy | (Bench-scale) | Tank rinse water | BCR | 36 | 48 | ∼100 | >85 | – | >95 | – | – | – | – | Capodaglio. & Callegari (2015) |
| Italy | (Pilot-scale) | Domestic wastewater | BCR | 90 | 48–60 | – | 93 | 37 | – | – | ∼51 | – | – | Capodaglio & Callegari (2016) |
| Greece | Toilet-flushing (Pilot-scale) | Single house wastewater | SMBR | – | – | – | 87 | 40 | 92 | ∼100 | – | 69 | ∼100 | Fountoulakis et al. (2016) |
| Egypt | (Pilot-scale) | Municipal wastewater | PABF | 120 | 3.5 | 93 | 91 | 66 | 97 | – | 87 | – | – | Abou-Elela et al. (2017) |
| Spain | Non-potable (Pilot-scale) | Graywater from hotel | MBR | 20–22 | 4 | 87 | 80–95 | 85 | – | – | 80 ± 32.2 | – | – | Atanasova et al. (2017) |
| USA | Wastewater | GFMBR | 20 | – | – | 93 | 46 | 60–77 | – | 99 | – | – | Platten et al. (2018) | |
| Egypt | (Pilot-scale) | Domestic sewage | Compact BCR | – | 12 | 85 | 80 | 48.5 | 90 | 97.5 | 52 | 65 | 97.6 | Aly Nasr et al. (2019) |
| China | Toilet-flushing (laboratory-scale) | Graywater | MBR (summer) (winter)BAF (summer) (winter) | 365365 | 2.5 3.02.36 5.89 | 90 8595 83 | 80 60–9084.4 78.6 | – – | 95 9590 80 | – – | 90 6090 5–60 | – – | – – | Ren et al. (2019) |
| Lithuania | Non-potable (Pilot-scale) | Wastewater | MWTU | 112 | 8 | 99.2 | 95.2 | 82.2 | 99.4 | – | 99.6 | 91.8 | – | Mažeikienė (2019) |
| Indonesia | Toilet-flushing (Pilot-scale) | STP | ABR | – | 52–138 | 74 | 70 | 43–84 | 68 | – | – | 21–90 | – | Yulistyorini et al. (2019) |
| Brazil | Non-potable (Pilot-scale) | Wastewater | Sand filter | 244 | 16.8 | 72 ± 10 | 77 ± 10 | 58 | 88 | – | – | – | – | de Oliveira et al. (2019) |
| Country | Application (scale) | Sampling location/type | Treatment scheme | SRT (days) | HRT (hours) | Removal (%) | Reference | |||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| BOD | COD | TN | TSS | TC (CFU/100 mL) | NH3-N | TP | FC | |||||||
| Germany | Non-potable (laboratory-scale/ BUSSE reactor) | Synthetic | SM-SBR | >250 | 13–60/33–100 | – | 79/89 | 37/41 | – | – | 99/98 | 50/71 | – | Gildemeister et al. (2005) |
| China | – | University | CABR | – | 48 | – | 79 | 19.22 ± 5.63 | 81.92 ± 3.53 | 77.35 | – | 30.86 ± 4.68 | 99.82 | Feng et al. (2008) |
| China | Toilet flushing (laboratory-scale) | Bathing wastewater | SMBR | 90 | 4 | 93.33 ± 1.03 | 87.57 ± 2.73 | 57.74 | 99.98 | – | 98.78 ± 0.21 | 90 | – | Jifeng et al. (2008) |
| Egypt | (Laboratory-scale) | Wastewater | USBR | >300 | 20 | 81 | 84 | – | 89 | – | _ | – | – | Sabry (2010) |
| Iran | (Pilot-scale) | Residential wastewater | UST | >350 | 24 | 85 | 77 | – | 86 | – | – | – | – | Moussavi et al. (2010) |
| Iran | Irrigation (Pilot-scale) | Synthetic sewage | ASR | >300 | 24 | 79.4 | 86.2 | – | 95 | – | – | – | – | Jorsaraei et al. (2014) |
| USA | (Laboratory-scale) | Synthetic wastewater | BCR 1. conventional 2. hybrid | 6 15 | 9 9 | – | 93 97 | 43–53 75–79 | – | – | – | – | – | Scott et al. (2013) |
| Spain | Toilet flushing (prototype) | Showers and bathrooms | MBR | – | 19.5 | 95 | 90 | – | 98 | – | – | – | – | Santasmasas et al. (2013) |
| London, UK | Non-potable (Pilot-scale) | Composite wastewater sample | Septic tank 1. conventional 2. baffled | – | 72 72 | 68.4 76.5 | 65.3 74 | 26.8 31.2 | 65.3 76 | – | – | 29.3 33.1 | 86 95 | Nasr & Mikhaeil (2013) |
| Iran | (Laboratory-scale) | STP | Bio-cache | 45 | 2 | 88 | 78 | – | 72 | 95 | 75 | 40 | 93 | Valipour et al. (2014) |
| Malaysia | Non-potable | Household | Aerobic digestion unit + H2O2 | – | 5 | – | 68 | – | 88 | 4 × 106 | – | – | – | Teh et al. (2015) |
| Jordan | Non-potable (laboratory-scale) | Graywater | SMBR | 42 | 7 | – | 88 | – | – | 29 | 8.98 | 56 | 26 | Bani-Melhem et al. (2015) |
| Italy | (Bench-scale) | Tank rinse water | BCR | 36 | 48 | ∼100 | >85 | – | >95 | – | – | – | – | Capodaglio. & Callegari (2015) |
| Italy | (Pilot-scale) | Domestic wastewater | BCR | 90 | 48–60 | – | 93 | 37 | – | – | ∼51 | – | – | Capodaglio & Callegari (2016) |
| Greece | Toilet-flushing (Pilot-scale) | Single house wastewater | SMBR | – | – | – | 87 | 40 | 92 | ∼100 | – | 69 | ∼100 | Fountoulakis et al. (2016) |
| Egypt | (Pilot-scale) | Municipal wastewater | PABF | 120 | 3.5 | 93 | 91 | 66 | 97 | – | 87 | – | – | Abou-Elela et al. (2017) |
| Spain | Non-potable (Pilot-scale) | Graywater from hotel | MBR | 20–22 | 4 | 87 | 80–95 | 85 | – | – | 80 ± 32.2 | – | – | Atanasova et al. (2017) |
| USA | Wastewater | GFMBR | 20 | – | – | 93 | 46 | 60–77 | – | 99 | – | – | Platten et al. (2018) | |
| Egypt | (Pilot-scale) | Domestic sewage | Compact BCR | – | 12 | 85 | 80 | 48.5 | 90 | 97.5 | 52 | 65 | 97.6 | Aly Nasr et al. (2019) |
| China | Toilet-flushing (laboratory-scale) | Graywater | MBR (summer) (winter)BAF (summer) (winter) | 365365 | 2.5 3.02.36 5.89 | 90 8595 83 | 80 60–9084.4 78.6 | – – | 95 9590 80 | – – | 90 6090 5–60 | – – | – – | Ren et al. (2019) |
| Lithuania | Non-potable (Pilot-scale) | Wastewater | MWTU | 112 | 8 | 99.2 | 95.2 | 82.2 | 99.4 | – | 99.6 | 91.8 | – | Mažeikienė (2019) |
| Indonesia | Toilet-flushing (Pilot-scale) | STP | ABR | – | 52–138 | 74 | 70 | 43–84 | 68 | – | – | 21–90 | – | Yulistyorini et al. (2019) |
| Brazil | Non-potable (Pilot-scale) | Wastewater | Sand filter | 244 | 16.8 | 72 ± 10 | 77 ± 10 | 58 | 88 | – | – | – | – | de Oliveira et al. (2019) |
SRT, sludge retention time; HRT, hydraulic retention time; COD, chemical oxygen demand; TN: total nitrogen; TP: total phosphorus; SM-SBR, submerged membrane – sequencing batch reactor; MBR, membrane bioreactor; CABR, carrier anaerobic baffled reactor; SMBR, submerged membrane bioreactor; USBR, upflow septic tank/baffled reactor; UST, upflow septic tank; ASR, advanced septic reactor; BCR, biomass concentrator reactor; STP, sewage treatment plant; PABF, passively aerated biological filter; BAF, biological aerated filter; MWTU, main (secondary) wastewater treatment unit; ABR, anaerobic baffled reactor; GFMBR, gravity-flow membrane bioreactor.
Performance inventory and filter characteristics of hybrid techniques for wastewater treatment reported in the literature
| Treatment process | Type of feed sample | Membrane characteristics | Experimental conditions | Pollutants | Removal (%) | Fouling decrease (%) | Reference |
|---|---|---|---|---|---|---|---|
| SMBR and PCR-DGGE | Bathing wastewater | Hollow fiber polyethylene MF, A = 3 m2, TMP = 4–33 kPa | HRT = 4 h, critical flux = 16 L/(m2.h), TMP = 4–33 kPa | – | – | – | Jifeng et al. (2008) |
| BCR | Groundwater | Ultra-high MWPE, d = 18–28 μm, A = 120 m2 Ultra-high MWPE, d = 20 μm, A = 45 m2 | Vol = 8 m3, aerobic Vol = 1.2 m3, aerobic | MtBE | >99 >99 | – | Capodaglio et al. (2010) |
| SMEBR | Synthetic wastewater | Hollow fiber membrane, Length = 0.2 m Pore size = 0.04 μm Cylindrical iron mesh cathode (A = 106 cm2) and anode (A = 93 cm2) | Tank Vol = 20 L, Working Vol = 13.4 L, voltage gradient = 1 V/cm, permeate flux = 9.72 L/(m2·h) | – | – | 16.3 | Bani-Melhem & Elektorowicz (2010) |
| SMBR + EC | Graywater from university | Two hollow fiber UF membranes Length = 0.2 m Pore size = 0.04 μm A = 0.047 m2 | Permeate flux = 29 L/(m2.h) Two aluminum electrodes were used DC supply = 12 V TMP = 7.5 kPa | COD Turbidity Color TSS TC FC NH3-N  | 89 97 94 ∼100 99.9 99.9 77.8 94.3 | 13 | Bani-Melhem & Smith (2012) |
| EC + MBR | Tannery wastewater | Hollow fiber MF membranes modules of PVDF were used, pore size = 0.1 μm, length = 0.344 m, TSA = 0.4 m2 | Operation period = 7 days, TMP = 5 kPa, Vol = 65.8 length, permeate flux = 34.5 L/(m2·h) | COD Color TSS | 94 97 100 | 8–12 | Vinduja & Balasubramanian (2013) |
| Ceramic filter + MBR | Synthetic graywater | Hollow cylindrical shape filter made up of clay soil + rice bran + water, H = 10 cm, do = 10 cm, di = 6 cm | Vol = 22 L, airflow rate = 4.5 L/min, permeate flux = 8.33 –11.75 L/(m2·h), HRT = 1.7 days | BOD5 TOC MBAS TSS | >97 >88 >99 >99 | – | Hasan et al. (2015) |
| AAO-MBR | University campus | 216 PVDF submerged hollow membrane fiber module Pore d = 0.1 μm, A = 5,400 m2 | Vol1 = 150 m3,Vol2 = 420 m3, Vol3 = 480 m3, Vol4 = 120 m3, HRT = 12.5 h, flow rate = 16 L/(m2.h) TMP = 10–30 kPa | – | – | 9 | Hu et al. (2016) |
| E2BCR | Urban wastewater | Stainless steel cylindrical wound wire, d = 24 mm, Length = 150 mm, pore size = 25μm | Aerated cylindrical reactor, Vol = 1.5 L Iron sacrificial at anode with A = 38 cm2 DC supply = 5 V, inflow rate- = 8 L/d, HRT = 4.5 h | COD | ∼90 | 25.2 | Cecconet et al. (2017) |
| Low- pressure GDMBR | Synthetic graywater | Flat sheet membrane made of polyethersulfone, A = 0.06 m2 | GDMBR1 (without aeration), Vol = 9 L, TMP = 50 mbar, permeate flux = 2 L/(m2.h) GDMBR2 (with aeration), Vol = 9 L, TMP = 50 mbar, permeate flux = 1 L/(m2.h) | COD (GDMBR1) COD (GDMBR2) | 94.5 96 | – | Ding et al. (2017) |
| E2BCR + coarser filter medium | Synthetic graywater | BCR built using a stainless filter (SS 316), slot size = 25 μm, d = 2.4 cm, H = 15 cm, TSA = 226.2 cm2 | Vol = 1.5 L, A = 37.5 m2, Inflow = 10 L/day, HRT = 3.6 h, anode (iron) and cathode (stainless steel) were used | COD | 92.45 | 30.4 | Cecconet et al. (2018) |
| Treatment process | Type of feed sample | Membrane characteristics | Experimental conditions | Pollutants | Removal (%) | Fouling decrease (%) | Reference |
|---|---|---|---|---|---|---|---|
| SMBR and PCR-DGGE | Bathing wastewater | Hollow fiber polyethylene MF, A = 3 m2, TMP = 4–33 kPa | HRT = 4 h, critical flux = 16 L/(m2.h), TMP = 4–33 kPa | – | – | – | Jifeng et al. (2008) |
| BCR | Groundwater | Ultra-high MWPE, d = 18–28 μm, A = 120 m2 Ultra-high MWPE, d = 20 μm, A = 45 m2 | Vol = 8 m3, aerobic Vol = 1.2 m3, aerobic | MtBE | >99 >99 | – | Capodaglio et al. (2010) |
| SMEBR | Synthetic wastewater | Hollow fiber membrane, Length = 0.2 m Pore size = 0.04 μm Cylindrical iron mesh cathode (A = 106 cm2) and anode (A = 93 cm2) | Tank Vol = 20 L, Working Vol = 13.4 L, voltage gradient = 1 V/cm, permeate flux = 9.72 L/(m2·h) | – | – | 16.3 | Bani-Melhem & Elektorowicz (2010) |
| SMBR + EC | Graywater from university | Two hollow fiber UF membranes Length = 0.2 m Pore size = 0.04 μm A = 0.047 m2 | Permeate flux = 29 L/(m2.h) Two aluminum electrodes were used DC supply = 12 V TMP = 7.5 kPa | COD Turbidity Color TSS TC FC NH3-N | 89 97 94 ∼100 99.9 99.9 77.8 94.3 | 13 | Bani-Melhem & Smith (2012) |
| EC + MBR | Tannery wastewater | Hollow fiber MF membranes modules of PVDF were used, pore size = 0.1 μm, length = 0.344 m, TSA = 0.4 m2 | Operation period = 7 days, TMP = 5 kPa, Vol = 65.8 length, permeate flux = 34.5 L/(m2·h) | COD Color TSS | 94 97 100 | 8–12 | Vinduja & Balasubramanian (2013) |
| Ceramic filter + MBR | Synthetic graywater | Hollow cylindrical shape filter made up of clay soil + rice bran + water, H = 10 cm, do = 10 cm, di = 6 cm | Vol = 22 L, airflow rate = 4.5 L/min, permeate flux = 8.33 –11.75 L/(m2·h), HRT = 1.7 days | BOD5 TOC MBAS TSS | >97 >88 >99 >99 | – | Hasan et al. (2015) |
| AAO-MBR | University campus | 216 PVDF submerged hollow membrane fiber module Pore d = 0.1 μm, A = 5,400 m2 | Vol1 = 150 m3,Vol2 = 420 m3, Vol3 = 480 m3, Vol4 = 120 m3, HRT = 12.5 h, flow rate = 16 L/(m2.h) TMP = 10–30 kPa | – | – | 9 | Hu et al. (2016) |
| E2BCR | Urban wastewater | Stainless steel cylindrical wound wire, d = 24 mm, Length = 150 mm, pore size = 25μm | Aerated cylindrical reactor, Vol = 1.5 L Iron sacrificial at anode with A = 38 cm2 DC supply = 5 V, inflow rate- = 8 L/d, HRT = 4.5 h | COD | ∼90 | 25.2 | Cecconet et al. (2017) |
| Low- pressure GDMBR | Synthetic graywater | Flat sheet membrane made of polyethersulfone, A = 0.06 m2 | GDMBR1 (without aeration), Vol = 9 L, TMP = 50 mbar, permeate flux = 2 L/(m2.h) GDMBR2 (with aeration), Vol = 9 L, TMP = 50 mbar, permeate flux = 1 L/(m2.h) | COD (GDMBR1) COD (GDMBR2) | 94.5 96 | – | Ding et al. (2017) |
| E2BCR + coarser filter medium | Synthetic graywater | BCR built using a stainless filter (SS 316), slot size = 25 μm, d = 2.4 cm, H = 15 cm, TSA = 226.2 cm2 | Vol = 1.5 L, A = 37.5 m2, Inflow = 10 L/day, HRT = 3.6 h, anode (iron) and cathode (stainless steel) were used | COD | 92.45 | 30.4 | Cecconet et al. (2018) |
GDMBR, gravity-driven membrane bioreactor; MWPE, molecular weight polyethylene; MtBE, methyl tertiary-butyl ether; SMBR + EC, submerged membrane bioreactor + electrocoagulation; E2BCR, electrically enhanced biomass concentrator reactor; PCR-DGGE, polymerase chain reaction–denaturing gel gradient electrophoresis; TMP, transmembrane pressure; AAO-MBR, anaerobic tank+ anoxic tank+ oxic tank+ MBR tank (full scale); PVDF, polyvinylidene difluoride; SMEBR, submerged membrane electro-bioreactor; MBAS, methylene blue active substance; EMR, electrocoagulation combined with microfiltration; EC + MBR, electrocoagulation integrated with membrane bioreactor; A, area; Vol, volume; d, diameter; do, outer diameter; di, inner diameter; H, height; TSA, total surface area.
Emerging and advance technologies for wastewater treatment
| Type of wastewater | Process conformation | Inoculants | Anode | Cathode | Separators | Treatment efficiency | Achieved voltage | Energy generation (power density/ current density) | Reference |
|---|---|---|---|---|---|---|---|---|---|
| Synthetic wastewater | BEMR, silicon tubes installed at both ends; column type reactor; HRT = 150 min, external resistor = 100 Ω, columbic efficiency = 8.2% | Concentrated anaerobic sludge | Granular graphite | Stainless steel mesh | Non-woven cloth (400 g/m2) | COD = 92.4% -N = 95.6% | 226–515 mV | 4.35 W/m3 18.32 A/m3 | Wang et al. (2011) |
| Synthetic wastewater | Integrated MFC-MBR system, resistor = 50 Ω, filter material = nylon mesh, silicon tubes inserted at top and bottom of anodic chamber., HRT = 40 days | Anaerobic + activated sludge from laboratory | Self-fabricated carbon fiber | Carbon felt | Non-woven fabric (400 g/m2) | COD = 89.6 ± 3.7% TSS ∼ 100% | 650 mV | 6.0 W/m3 | Wang et al. (2012) |
| Synthetic wastewater | MBER with GAC. consisted of tubular reactor of cation exchange PVDF hollow fiber membrane, resistor = 48 Ω, HRT = 19.6 h, TMP = <30 kPa | Anaerobic digester sludge | Carbon cloth supported with stainless steel mesh | One layer carbon cloth coated with platinum | Titanium wires | COD = 91.6% TCOD = 95.0% TSS = >80.0% | – | 1.8 W/m3 16.4 A/m3 | Li et al. (2014) |
| Synthetic + fresh wastewater | Overflow-type EMBR, anode and cathode chambers connected by overflow channel, resistor = 100 Ω, HRT = 16.9–8.5 h | Activated sludge | O-ring carbon felt | Stainless steel mesh | Titanium wires | COD = 92.6% TN = 73.9%  -N = 96.5% | 250.3 mV OCV = 840–861 mV | 629 mW/m3 | Zhou et al. (2015) |
| Synthetic wastewater | MBER, tubular reactor of anion exchange PVDF hollow fiber UF membrane, GAC coating at cathode, resistor = 10 Ω, HRT = 6 h, TMP = 10–15 kPa, | Anaerobic sludge | Carbon cloth | Carbon cloth coated with Pt/C powder | Epoxy | TCOD = 73.2% | – | 256.0 A/m3 | Li et al. (2016) |
| Synthetic wastewater | BES-DEN: Ag/AgCl worked as a reference electrode (+0.197 V vs. SHE), internal resistance = 33 Ω, medium = granular, operation period = 27 days | Activated sludge | Graphite rod | Stainless steel rod | Cation exchange membrane | Nitrate >90% | – | – | Molognoni et al. (2017) |
| Medicine wastewater + tap water | ABR-BEF: reactor was made up of polymethyl methacrylate, HRT = 24 h, DO conc. = 4 mg/L, resistance = 50 Ω | Anaerobic sludge | Fiber carbon (Toray carbon filament + titanium wire) | Hydrophilic carbon cloth | Proton exchange membrane | Catechol >99.7% COD >91.7% | 424.9 mV | 77.1 mW/m3 | Su et al. (2019) |
| Type of wastewater | Process conformation | Inoculants | Anode | Cathode | Separators | Treatment efficiency | Achieved voltage | Energy generation (power density/ current density) | Reference |
|---|---|---|---|---|---|---|---|---|---|
| Synthetic wastewater | BEMR, silicon tubes installed at both ends; column type reactor; HRT = 150 min, external resistor = 100 Ω, columbic efficiency = 8.2% | Concentrated anaerobic sludge | Granular graphite | Stainless steel mesh | Non-woven cloth (400 g/m2) | COD = 92.4%-N = 95.6% | 226–515 mV | 4.35 W/m3 18.32 A/m3 | Wang et al. (2011) |
| Synthetic wastewater | Integrated MFC-MBR system, resistor = 50 Ω, filter material = nylon mesh, silicon tubes inserted at top and bottom of anodic chamber., HRT = 40 days | Anaerobic + activated sludge from laboratory | Self-fabricated carbon fiber | Carbon felt | Non-woven fabric (400 g/m2) | COD = 89.6 ± 3.7% TSS ∼ 100% | 650 mV | 6.0 W/m3 | Wang et al. (2012) |
| Synthetic wastewater | MBER with GAC. consisted of tubular reactor of cation exchange PVDF hollow fiber membrane, resistor = 48 Ω, HRT = 19.6 h, TMP = <30 kPa | Anaerobic digester sludge | Carbon cloth supported with stainless steel mesh | One layer carbon cloth coated with platinum | Titanium wires | COD = 91.6% TCOD = 95.0% TSS = >80.0% | – | 1.8 W/m3 16.4 A/m3 | Li et al. (2014) |
| Synthetic + fresh wastewater | Overflow-type EMBR, anode and cathode chambers connected by overflow channel, resistor = 100 Ω, HRT = 16.9–8.5 h | Activated sludge | O-ring carbon felt | Stainless steel mesh | Titanium wires | COD = 92.6% TN = 73.9% -N = 96.5% | 250.3 mV OCV = 840–861 mV | 629 mW/m3 | Zhou et al. (2015) |
| Synthetic wastewater | MBER, tubular reactor of anion exchange PVDF hollow fiber UF membrane, GAC coating at cathode, resistor = 10 Ω, HRT = 6 h, TMP = 10–15 kPa, | Anaerobic sludge | Carbon cloth | Carbon cloth coated with Pt/C powder | Epoxy | TCOD = 73.2% | – | 256.0 A/m3 | Li et al. (2016) |
| Synthetic wastewater | BES-DEN: Ag/AgCl worked as a reference electrode (+0.197 V vs. SHE), internal resistance = 33 Ω, medium = granular, operation period = 27 days | Activated sludge | Graphite rod | Stainless steel rod | Cation exchange membrane | Nitrate >90% | – | – | Molognoni et al. (2017) |
| Medicine wastewater + tap water | ABR-BEF: reactor was made up of polymethyl methacrylate, HRT = 24 h, DO conc. = 4 mg/L, resistance = 50 Ω | Anaerobic sludge | Fiber carbon (Toray carbon filament + titanium wire) | Hydrophilic carbon cloth | Proton exchange membrane | Catechol >99.7% COD >91.7% | 424.9 mV | 77.1 mW/m3 | Su et al. (2019) |
BEMR, bio-electrochemical membrane reactor; MFC-MBR, microbial fuel cell – membrane bioreactor; MBER, membrane bio-electrochemical reactor; EMBR, electro-chemical membrane bioreactor; BES-DEN, bio-electrochemically based denitrification reactor.; ABR-BEF, anaerobic baffled reactor-bio-electricity-Fenton; TCOD, total chemical oxygen demand.
Table 2 summarizes performance parameters of some of the physical and biological techniques for the treatment of decentralized wastewater implemented all over the world, and each technique/process is presented in relation to removal of various wastewater parameters including the elimination of the targeted contaminant.
Table 3 summarizes the performance inventory and filter characteristics of some hybrid techniques for wastewater treatment systems reported in literature and these composite technologies reduce the fouling problem and increase the system efficiency. These technologies are effective to provide treated effluents that can meet the discharge standards as mentioned in Table 1.
Table 4 summarizes the emerging and advanced technologies for wastewater treatment that are very useful for both wastewater treatment and generation of energy. The technology includes bio-electrochemical systems (BES), microbial fuel cells (MFCs), etc., which recover energy from wastewater containing microorganisms or organic matter. BES are the emerging wastewater treatment systems which can convert the chemical energy of wastewater into electrical energy by the action of electrochemically active microorganisms (Jain & He 2018; Yang et al. 2019). These systems consist of an anode and a cathode where reduction/oxidation of the chemicals (organic/inorganic) present in the wastewater takes place and electricity is also produced simultaneously (Pant et al. 2011; Wang et al. 2015; Xu et al. 2019). There are different types of BES available in literature; MFCs are the most commonly applied BES in wastewater treatment. MFCs are a kind of BES that use wastewater as food to microorganisms in wastewater treatment, bioremediation, bio-hydrogen production, carbon capture, bio-sensing and bio-electricity generation (Sun et al. 2015; Singh et al. 2019). The research is still going on to improve the performance of these technologies on a wide- or full-scale wastewater treatment.
DISCUSSION AND PERSPECTIVES
There are a variety of technologies available in literature that have been successfully implemented for treating various forms of decentralized wastewater. The performance and size of the reactor provides a good comparative study of various types of treatment process, but still it is very difficult to compare their economic affordability, as the cost-effectiveness of these treatment processes is based on the varieties of materials used and local economic conditions. The following outcomes were drawn from the tables that are mentioned in this review.
From Table 2, MBRs and BCRs proved to be the most efficient treatment technologies among other treatment methods with chemical oxygen demand (COD) removal of more than 90% in most of the cases. The most consistent technology in removal of COD and total suspended solids (TSS) was found to be BCR, with removal efficiency around 93% and 95% respectively (Scott et al. 2013; Capodaglio & Callegari 2015, 2016; Aly Nasr et al. 2019). The increase in overall performance and less land requirement, compared with, for example, constructed wetlands, make membrane systems a promising technique to treat DWW efficiently at source (Wu et al. 2015; Wang et al. 2017). MBRs were first introduced in the 1960s; however, they have gained increased attention in the early 21st century. Numerous advantages like smaller space requirement, high sludge retention time (SRT), reduced microbial contamination, biomass waste control and excellent effluent quality make MBRs more desirable than conventional treatment systems (Le-Clech 2010; Shin & Bae 2018; Cecconet et al. 2019a). Both MBRs and BCRs showed comparable performance in removing COD, ammonia nitrogen and TSS from the wastewater/graywater, but due to complexity and cost of MBRs operation, BCRs are preferred over MBRs as BCRs utilize high-porosity filters and have lower energy demands than MBRs and are capable of producing better quality effluent than MBRs (Capodaglio et al. 2010; Cecconet et al. 2019b). However, there is still the need for further research on control of microbial quality obtained from BCRs effluent. Also, membrane fouling and requirement for frequent cleaning add to the cost of treatment, which makes membrane treatment systems uneconomical and there is a need for advancement in the process configuration.
There have been new strategies made to control fouling issues and to increase the pollutant removal efficiency in MBRs and BCRs by combining the two technologies (hybrid systems) and introduction of electric fields in the reactors (Neoh et al. 2016) (Table 3). The electrocoagulation process combined with membrane technologies was proved to reduce fouling problem in membranes. Electrically enhanced biomass concentrator reactor (E2BCR) technology applied by some researchers for urban wastewater and synthetic graywater treatment was not only effective for COD removal of 90% and 92.45%, but also the issue of membrane fouling was significantly reduced by up to 25.2% and 30.4% respectively (Ahmed et al. 2017; Cecconet et al. 2017, 2018). The hybrid technologies not only reduced the fouling of membrane but also were more efficient in removing COD, TSS, total coliforms, fecal coliforms and biochemical oxygen demand etc. than conventional membrane treatment systems (CMTS). A hybrid system containing a ceramic filter of pore size 1–5 μm with an MBR for a 6 month period was found to be the most effective system in obtaining removal efficiency of 99%, 97%, 88% and 99% for methylene blue active substance (MBAS), BOD5, total organic carbon and TSS respectively; also membrane fouling susceptibility and cost to reuse wastewater were reduced (Hasan et al. 2015). Although hybrid systems are advantageous over CMTS, still the problems like excess sludge production (Vinduja & Balasubramanian 2013), inefficient removal of nitrogen (Bani-Melhem & Smith 2012; Cecconet et al. 2017) and removal of potentially hazardous micro-pollutants have yet to be addressed.
Table 4 presents the emerging and advanced technologies like bio-electrochemical systems (BES), MFCs and Fenton in which microorganisms oxidize organic compounds, and electrons generated by them are thus utilized for the production of energy and other valuable compounds (Wang & Ren 2013; Kumar et al. 2019; Singh et al. 2019). The previous mentioned limitations regarding removal of nitrogen and micro-pollutants were significantly improved by adopting advanced treatment methods coupled with various physical and biological processes. Researchers have successfully designed several hybrid system such as MBR-MFC, which can remove up to 89.6 ± 3.7% of soluble COD and approximately 100% of TSS from wastewater, with a carbon cathode, providing power density of 6.0 W/m3 (Wang et al. 2012). A BES-DEN (bio-electrochemically based denitrification reactor) reactor was also applied for removing nitrate by up to 90% by using a stainless steel rod and graphite rod at the cathode and anode respectively (Molognoni et al. 2017). An overflow-type EMBR (electro-chemical membrane bioreactor) which uses stainless steel mesh and O-ring carbon felt at the cathode and anode was capable of removing
-N, total nitrogen and COD by 96.5%, 73.9% and 92.6% respectively (Zhou et al. 2015). Another reactor made up of poly-methyl methacrylate named as ABR-BEF showed 91.7% removal efficiency for COD and as high as 99.7% removal for the targeted micro-pollutant (catechol), and has power density output of about 77.1 mW/m3 (Su et al. 2019).
The integration of several biological, electrochemical and advanced oxidation processes (AOPs) has proven to be effective in the treatment of different types of wastewater (Ahmed et al. 2017; Cecconet et al. 2017). Therefore it can be inferred from the literature that the integration of advance treatment technology (MFCs, BES, AOPs, etc.) with the conventional physical, biological or membrane processes proves to be constructive and effective in the treatment of various pollutants including micro-pollutants from wastewater/ graywater and in the generation of bio-electricity, which makes them the best sustainable technology at present and be suitable for future generations.
CONCLUSION AND FUTURE SCOPE
The membrane-based technologies applied in wastewater treatment as well as in graywater treatment guarantee superior quality of effluent which ensures graywater reuse guidelines for non-potable purposes are met. However, optimization of fouling mechanisms, removal and recovery of nutrients, addition of disinfection units and life cycle assessment in membrane-based technology have yet to be investigated.
The available advanced BES technology was only implemented at bench scale and more investigation is needed to implement these technologies at wide scale and industrial level.
There was plenty of data available about microbial interactions of different electro-chemical materials (cathode and anode) but these materials are not eco-friendly and economical: thus further research is required to explore sustainable materials like biomaterials, non-porous and lithospheric materials that can easily decompose in the surroundings.
From the study it can be inferred that integration of membrane technology with MFCs is the most effective technology in treating wastewater among all discussed methods but the attention now should be given to treating wastewater containing pollutants like dyes, leachate, heavy metals and sulfate. Also there is a need for further research in integration of MFCs with chemical/physical as wells as with biological techniques like aerobic/ anaerobic degradation, and in the development of MFCs a power storage system is a must.
Hybrid technologies like E2BCR and integration of physical/biological methods with BES such as MFCs were found to be the most promising methods for near complete removal of pollutants from wastewater and also the issue of membrane fouling was reduced to a good extent.
Several studies on BCRs technology show that BCRs are much simpler and economical and ensure better effluent quality than MBRs. BCRs also showed great potential in reducing the fouling of membranes. Therefore much wider application of BCRs at industrial scale should be investigated for the treatment of graywater, multicomponent wastewater and pharmaceutical wastewater in a decentralized manner. BCRs can be coupled with MFCs, AOPs, BES or other advanced technology to further increase the process efficiency for wastewater treatment in future.
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