Schematic diagram of an up-flow Anaerobic Sludge Blanket Reactor (UASBR).
2.9. Aerobic–Anaerobic Systems
Aerobic–anaerobic systems have been widely investigated for various wastewaters with high organic contents. The combination of these two technologies allows for a more complete remediation of DWW as each stage focuses on different pollutants within the wastewater. The aerobic stage effectively reduces the ammonia, phosphorous, H
25 content, whereas the anaerobic stage reduces the COD and nitrate content of the water [21]. In theory, it should be possible to achieve removal yields from a two-stage system similar to the yields that can be achieved by the individual processes. This is due to the different pollutants that are being processed by each individual system. In a practical study of an anaerobic filter combined with an aerobic one, an SBR was able to achieve a COD removal of 90% from a high-strength dairy stream with an influent COD content of approximately 8000 g/L [27].
content, whereas the anaerobic stage reduces the COD and nitrate content of the water [3]. In theory, it should be possible to achieve removal yields from a two-stage system similar to the yields that can be achieved by the individual processes. This is due to the different pollutants that are being processed by each individual system. In a practical study of an anaerobic filter combined with an aerobic one, an SBR was able to achieve a COD removal of 90% from a high-strength dairy stream with an influent COD content of approximately 8000 g/L [13].
3. The Effects of Experimental Parameters on the Performance of Biological Treatment Systems
There are three main groups of microorganisms which are found in AD: acidogenic (acid producing), acetogenic (acetate-producing), and methanogenic (methane-producing). Each of these has a pH range which it prefers. Acidogens have an optimal pH range between 5.2 and 6.5; acetogens from 6.6 to 7.6; and methanogens from 7.5 to 8.5
[38][14]. The pH of the system should be carefully controlled because an imbalance, either too high or too low, can lead to an overproduction or accumulation of certain compounds, which can then result in digestor failure
[39][15]. It is important to select an appropriate pH based on the specific need of the system. However, the reactions which occur during the remediation of DWW produce volatile fatty acids (VFAs) from the breakdown of FOGs, which alter the pH as the process occurs.
3.2. Dissolved O
2
Levels
Aerobic digestion is enhanced by higher levels of dissolved O
2; it therefore follows that anaerobic digestion will be enhanced by the removal of dissolved O
2 from the wastewater. One straightforward approach to eliminate dissolved oxygen is to pass another gas through the wastewater. Some potential deoxygenating agents for anaerobic digestion are nitrogen or biogas. Biogas is a convenient option as an amount of CO
2 and methane is produced by anaerobic digestion. However, the use of biogas poses an environmental issue due to methane being a greenhouse gas. Nitrogen gas is a suitable gas to use as it is non-reactive and does not pose any inherent environmental risks
[39][15].
3.3. Hydraulic Retention Time
Hydraulic retention time (HRT) refers to the average time that a liquid substrate will spend within a reactor. HRT is largely dependent on the type of reactor being used and the organic load of the wastewater being treated. A small HRT can sometimes result in high biomass washout, whereas a long HRT can require large reactor volumes
[39][15]. The growth of the various groups of microorganisms used in AD are favoured by different HRTs. Methanogens prefer a longer HRT period as opposed to acidogenic bacteria, which prefer low HRTs. The determination of optimal hydraulic retention time (HRT) is a complex decision that requires the consideration of both the process and the desired outcomes, and it should be evaluated on a case-by-case basis.
3.4. Aeration and Agitation
An important factor in aerobic treatments is the aeration which is introduced into the system. It is the driving force behind the reactions which aerobic digestion is utilized for, namely, the oxidation of ammonia and the breakdown of complex organic compounds. Therefore, an optimal aeration regime is essential for the performance of any treatment method used. At lower aeration levels, it was observed that there was a significant decrease in COD removal efficiency and an even more drastic decrease in the efficiency of ammonia removal
[40][16]. This indicates that the available oxygen is being used to oxidise the carbon substrates in the wastewater as opposed to the ammonia. The most likely explanation is that ammonia requires a larger amount of O
2 when compared to the organic components.
3.5. Temperature
The temperature plays a critical role in the reactions involved in wastewater remediation. Different temperature ranges can have both favourable and unfavourable effects on the performance of a bioreactor. High temperatures can cause the denaturation of proteins within the microbes, causing them to lose their enzymatic activity and die. Low temperatures can cause the microbes to become dormant, inactive, or die. It is, therefore, essential that the temperatures within a bioreactor are carefully controlled and are not allowed to go beyond the range within which the microbes are able to thrive. However, choosing an optimal temperature for which to run a system at depends on the specific microorganisms used within it. Most anaerobic digesters used for wastewater treatment typically operate at mesophilic conditions, between 35 °C and 37 °C. Temperature needs to be carefully controlled as large fluctuations can be detrimental for these processes
[44][17].
4. Accessory Treatment Options
4.1. Hydrolysis
Anaerobic systems generally have difficulty in degrading FOGs and ammonia, as these are oxidation reactions which occur. High concentrations of FOGs can contribute to clogging within the reactor as they are poorly broken down through AD. An effective solution to these problems is a hydrolysis stage before the anaerobic digestor. An up-flow ASB reactor (UASB) was observed to be able to effectively treat wastewaters with high FOG contents which had been hydrolysed before treatment. It was noted that when fed unhydrolyzed wastewater, the UASB showed unstable COD removal and a tendency to accumulate some of the fats within the sludge
[45][18]. This suggests that the inclusion of a separate hydrolysis stage can mitigate the risk of clogging within the reactor.
4.2. Coagulation
Coagulation/Flocculation are currently the most commonly used processes for the removal of suspended and dissolved solids, colloids, and organic components in industrial wastewater
[47][19]. There are two types of coagulants commonly used in wastewater treatment, namely, inorganic and organic coagulants. Inorganic coagulants are usually metal-based salts, usually containing aluminium or iron. The use of Alum (X × Al(SO
4)
2 × 12H
2O, where X is a metal ion such as potassium or sodium) as a coagulant in the treatment of DWW was observed to reduce the turbidity of the water by 95% and reduce COD by 68%
[48][20]. This was further enhanced by the addition of polymeric coagulants, resulting in a reduction in COD by 85%. However, inorganic coagulants produce large amounts of metal-rich floc, which must be further treated before it is disposed of. They can also alter the pH of the water requiring pH control and corrosion-resistant equipment. When trying to integrate inorganic coagulants with biological systems, it is important to evaluate whether the microbial colonies will be able to function unhindered.
Organic coagulants can either be polymeric or natural coagulants. Polymeric coagulants produce longer polymer chains without any metals or hydroxides and produce smaller volumes of floc. They also do not impact the pH of the water. They also produce low-density floc, which does not always settle well. Natural coagulants are being investigated widely and there have been many studies which show their effectiveness in the treatment of wastewater. An extract made from the bark of
Guazuma ulmifolia was used as a coagulant for DWW (3037 mg
COD/L & 1283 mg
BOD5/L) and was observed to remove 95.8% of the turbidity, 76% of the COD and 81.2% of the BOD
5 [49][21]. Natural coagulants’ low cost and eco-friendliness make them a suitable alternative to the more commonly used synthetic coagulants.
4.3. Membrane Filtration
Membrane filtration methods such as microfiltration are effective at significantly reducing the TSS within the wastewater. However, it has little effect on the TN, COD, and BOD
5 content and is commonly used as a pre-treatment step in a wastewater treatment process. Reverse osmosis (RO) is a viable option for dairy wastewater treatment and has been observed to reduce the TN and TOC by 94% and 84%, respectively
[50][22]. Nanofiltration has been observed to be effective at reducing COD and TSS levels but does not remove all ions of interest, such as phosphates and nitrates, from the wastewater stream
[51][23]. There have been some applications of nanofiltration and RO being used in conjunction with a bioreactor, which was observed to greatly improve the overall ion removal in addition to enhancing the reduction in COD and TSS. There are issues associated with RO and membrane filtration, which affect the long-term uses of this type of system. RO is an extremely energy-intensive process and is sometimes impractical to operate at the specifications required
[52][24]. The replacement of this technology can be costly in the event of physical damage or fouling. In addition, this process produces a highly concentrated retentate which needs to be disposed of. This is a major concern as the disposal of the concentrate that is formed is more problematic than that of the wastewater itself.