3.3. Chemical Treatment
Chemical treatment processes involve pH adjustment or coagulation/flocculation by adding different chemicals to the effluent to alter its chemistry [
11]. Coagulation-flocculation is the first treatment step in the chemical wastewater treatment method. Flocculation involves stirring/agitation of chemically-treated effluent to induce coagulation that improves sedimentation performance by increasing particle size, thereby increasing settling efficiency [
15]. Inorganic coagulants such as aluminum sulfate and ferric chloride have been widely applied in wastewater treatment [
16]. During this treatment process wastewater, organic compounds are oxidized via the addition of chemical compounds like chlorine, ozone-oxygen, or permanganate to generate CO
2, H
2O and other inoffensive materials [
17]. Chemical flocculants are highly efficient but are dangerous to human health and the environment [
17]. The wastewater pH needs to be maintained between 6 and 9 in order to protect the microorganisms (bacteria) present. Usually, neutralization of wastewater pH using H
2SO
4 and HCl is not recommended due to their corrosive nature and the discharge limitation of sulfate and chloride [
11]. However, the waste CO
2 could be utilized as an acidifying agent to decrease alkalinity (high pH) of wastewaters before the anaerobic digestion. The
Detarium microcarpum is reported to be an effective bio-coagulant for removal of turbidity from brewery effluent [
16]. Okolo et al. [
18], conducted a study on optimizing bio-coagulants for brewery wastewater treatment using response surface methodology. The method was used to evaluate the effects and the interactioin of three factors i.e., coagulants dosage, pH and the stirring time for solid particle removal on the treatment efficiency using
Detarium microcarpum seed powder (DMSP) and oyster dried shell powder (ODSP) as coagulants. The results demonstrated the optimum conditions for coagulant dosage (100.53 mg/L), effluent pH (2.001) and stirring time (24.47 min) with 90.44% solid particle (SP) removal for DMSP and coagulant dosage (104.19 mg/L), pH (3.34) and stirring time (27.54) with 96.55% SP removal for ODSP.
3.5. Membrane Filtration
The membrane filtration process uses semi-permeable materials that allow certain molecules to pass through them. Membrane filtration techniques such as reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF) and microfiltration (MF) have been applied significantly in brewery effluent remediation and can result in 99% removal of COD, BOD, and TSS [
2,
31]. Efficient membranes are characterized by high pollutant rejection rates, great durability, high permeate flux, low maintenance cost and high resistance to chemicals [
32]. Membrane filtration is considered safe and environmentally friendly [
33]. Membrane filtration has been known for its efficient removal of physical, microbial and chemical pollutants compared with other systems, hence it forms an integral part of the drinking water treatment process [
34].
NF was used by Braeken et al. [
2] to treat brewery wastewater for recycling; the results of the study showed that NF efficiently removed COD, Na
+, and Cl
− with an average removal rate of 100%, 55%, and 70%, respectively. Also, several studies on the application of RO reviewed by Madaeni and Mansourpanah [
35], revealed that RO may decrease the COD of the effluent by more than 90% or even completely. Madaeni and Mansourpanah [
35] biologically treated alcohol wastewater (which is similar to that of brewery wastewater since both productions involved fermentation) from a manufacturing plant by various polymeric RO and NF membrane with COD range of 900 to 1200 mg·L
−1nv. A polyethylene terephthalate RO membrane yielded magnificent results with higher flux (33 kg m
−1 h
−1) while COD was completely removed (100%). In another test conducted on brewery bio-effluent, using an internal aerobic membrane bioreactor (internal MEMBIOR), the effluent’s COD varied strongly (from 1500 to 3500 mg·L
−1) during the treatment process but was later reduced to about 30 mg·L
−1 at the end of the treatment. The membrane also retained the suspended solid completely making the effluent suitable for reuse [
36]. The main challenges for this technology are fouling and high energy consumption. More research should be focused on anti-fouling and less energy consumption membrane filtration methods for efficient treatment of brewery effluent.
3.6. Membrane Bioreactor Treatment
A membrane bioreactor treatment (MBR) is the combination of two treatment technologies that are membrane filtration and advanced biological treatment (activated sludge or an anaerobic unit). This technology has produced positive results in wastewater treatment over the past decade [
37,
38].
Increasing water prices and scarcity has called for combination of treatment technologies that can effectively treat wastewater for its reuse. MBR is noted as an economical and technically feasible choice of wastewater treatment [
39]. Descriptively, MBR is a system that integrates membrane with a bioreactor. Submerged and side-stream configurations are the two main recognized MBR systems. In a submerged process, the membrane is placed inside the reactor and then submerged into mixed liquor. In a sidestream process, the membrane unit is positioned outside the reactor and the reactor mixed liquor flows over a recirculation loop containing the membrane. However, sidestream MBRs are more energy intensive than submerged MBRs due to their high operational transmembrane pressures and the substantial volumetric flow needed to attain the preferred cross flow velocity [
40].
The technology was applied by Dai et al. [
41] in brewery wastewater treatment using an upflow anaerobic sludge blanket (UASB) reactor with an integrated membrane. The result showed successful removal of COD by 96%. There are extensive published articles on the application of MBR technology in brewery wastewater treatment with almost all reporting significant levels of COD removal rate by 90% or more [
42,
43,
44]. This has demonstrates that the MBR process can be an effective method for brewery wastewater treatment. Nevertheless, MBR technology is challenged by factors such as (1) fouling that needs to be addressed by regular cleaning and maintenance, (2) high capital cost due to the combination of more than one treatment methods (membrane and aerobic/anaerobic reactors), (3) high energy consumption leading to extra costs.
3.7. Advanced Oxidation Treatment Process
Advanced oxidation treatment processes (AOPs) are widely used in the treatment of both distillery and brewery wastewater. In this process, hydroxyl radicals (•OH) are produced by the use of ozone, hydrogen peroxide and ultraviolet irradiation in the first stage of the oxidation. In the second stage, organic loads react with hydroxyl radicals to produce precipitates. AOP technologies can be made possible through the combination of the hydrogen peroxide/ultraviolet irradiation (H
2O
2/UV), zone/ultraviolet irradiation (O
2/UV) and ozone/hydrogen peroxide (O
2/H
2O
2) [
45]. Ozone and hydroxyl radicals (•OH) are robust oxidants and can oxidize many organic compounds. Ozone reacts with an appreciable number of organic compounds when dissolved in water, thereby aiding in the removal of removal of organic contaminants from wastewater. It reacts directly or indirectly in the oxidation process, directly as molecular ozone and indirectly by the production of secondary oxidants in the form of free radical species such as hydroxyl radicals (•OH) [
45]. Fenton’s oxidation is another known AOP process based on the Fenton reaction. This process is a combination of hydrogen peroxide/ion salts (Fe
2+ or Fe
3+) [
46]. Fenton oxidation technology produces hydroxyl radicals (•OH) that result in precipitate formation and decolorization of effluent. Fenton technology produces a homogeneous reaction that is ecologically friendly [
46]. A further search on AOP processes showed a few other combination such as TiO
2/U, boron-doped diamond electrodes and catalytic ozonation. However these processes are still on laboratory scale utilization stage. Application of AOP in brewery and other wastewater treatment showed positive results and have the potential for future brewery wastewater treatment. However the technology may require supplementary treatment to eliminate ozone and this may increase the treatment cost. Also, the AOP processes are challenged by turbidity and NO
3 which needs to be addressed.
3.8. Air Cathode Microbial Fuel Cells Treatment
Recently, microbial fuel cell (MFC) wastewater treatment method has drawn worldwide attention due to its potential to convert organic pollutants into electricity whilst simultaneously purifying effluent. MFC reactors are combined with anaerobic treatment characteristics; that is using microorganisms to digest organic pollutants close to the anode, with the cathode exposed to oxygen. Electrons (released by bacterial oxidation of the organic loads) are transferred via the external circuit to the cathode, where they combine with oxygen to form water [
47]. For example, Feng et al. [
47], evaluated the efficiency and suitability of the MFC process in brewery wastewater treatment. According to the authors, for an effective MFCs process, there is the need for a good understanding of the how operational factors and the solution chemistry influence treatment efficiency. Furthermore, the authors evaluated the efficiency of MFC by examining maximum densities, and removal of COD as functions of effluent strength, temperature and columbic efficiencies (CEs). The result showed a reduction of maximum power density from 205 mW/m
2 to 170 mW/m
2 when the temperature was reduced from 30 °C to 20 °C. Nevertheless, there was slight decrease in COD removal and CEs with decreasing temperature. Moreover, the buffering capacity strongly affected the efficiency of the rector. COD removal rate was 85% at 20 °C and 87% at 30 °C. This technology can be used as a new method of brewery effluent treatment.
3.10. Microalgae Treatment Method
Microalgae are considered to be one of the favorable wastewater agents due to their ability to absorb nutrients and convert them to biomass [
50]. During the brewery wastewater treatment, nitrogen, phosphorus and other nutrients present in the wastewater are adequately absorbed by microalgae for their growth. Microalgae, through their photosynthetic activities, freely release oxygen which is utilized by bacteria in the wastewater. Microalgae also fix CO
2 by assimilating HCO
3 from CO
2 via respiration.
Figure 1 shows the mechanism of the bacteria-microalgae relationship in wastewater.
Until recently, the application of microalgae in wastewater treatment had only been restricted to the laboratory. Raceway ponds and photobioreactor technologies have been applied in microalgae wastewater treatment, including brewery wastewater. Raceway ponds are semi-circular at the two ends, with a shallow open system. The system has paddle wheels that provide continuous mixing of the microalgae in the wastewater for nutrients and sunlight [
51]. A raceway pond is depicted in
Figure 2. Photobioreactors are constructed either in vertical and horizontal columns. The structure allows penetration of light to the microalgae. CO
2 is sparged in and circulated to allow microalgae to have access to enough CO
2 [
52].
Figure 3 shows the model of the tubular photobioreactor.
A study by Lutzuet et al [
53] demonstrated that microalgae (
Scenedesmus dimorphus) was able to remove more than 99% of both nitrogen (N), and phosphorous (P) from brewery wastewater within one week; nitrogen was reduced from the initial concentration of 229 mg·L
−1 to a final concentration below 0.2 mg·L
−1 and phosphorous initial range of 1.4–5.5 mg·L
−1 to the final concentration lower than 0.2 mg·L
−1.
Similarly, Ferreira et al. [
54] concluded in their report that
Scenedesmus obliquus removed almost all the pollutants present in the various wastewater (poultry, swine and cattle breeding, brewery and dairy industries, and urban). Subramaniyam et al. [
55] cultivated
Chlorella sp. in brewery wastewater and concluded that
Chlorella sp. removed total nitrogen, phosphorus, and organic carbon completely with substantial growth of the microalgae (
Chlorella sp.).
Luo et al. [
56] also determined the nutrient removal efficiency by
Desmodesmus sp. CHX1 in piggery wastewater and reported that
Desmodesmus sp. CHX1 removed 78.46% of nitrogen and 91.66% of phosphorus. Another study conducted by Duan et al. [
57], compared the biochemical compositions of four microalgae (
Nannochloropsis oceanica,
Auxenochlorella pyrenoidosa,
Arthrospira platensis, and
Schizochytrium limacinum) and four macroalgae (
Ulva prolifera,
Saccharina japonica (
Areschoug),
Zostera marina, and
Gracilaria eucheumoides Harvey) and arrived at the conclusion that, the nitrogen and phosphorus contents in the algal biomass ranged from 1.24 to 10.79% and 0.03% to 2.49%, respectively, and confirmed the nutrient absorption capability of microalgae from wastewater. Travieso et al. [
58], conducted a study on the efficacy of distillery effluent treatment by the microalga
Chlorella vulgaris. The authors concluded that
Chlorella vulgaris reduced more than 98% of COD and BOD, and the final effluent was safe to be discharged into the environment.
Though the microalgae method is capable of removing high amount of pollutants from brewery effluent, the technology is limited in terms of salt, odor and color removal. This technology requires a combination with other cost effective method(s) for total removal of contaminants from the effluent. Microalgae-based wastewater treatment technology can be combined with membrane technology for the polishing treatment stage. Light and temperature are one of the limiting factors of algae-based wastewater treatments. Microalgae require optimum light and temperature for growth. This technology may therefore not be applicable in temperate regions due to the relatively low sunlight and temperature. Alternatively, artificial lighting systems could be used in these countries which may increase the cost of treatment. However, the biomass can be processed into biofuel and other useful products.