Effectiveness of an Upflow Anaerobic Sludge Blanket Reactor for Treating Dairy Wastewater

Wastewater production has increased as a result of urbanization and global population growth, since there is less space to build more wastewater treatment facilities. Furthermore, more stringent rules have been implemented to restrict the release of different contaminants; thus, it is essential to create new and different technologies for wastewater treatment [1]. One of the industries that generates wastewater that is high in organic matter is the dairy industry, which results in the generation of water that is foul-smelling and includes a lot of chemical oxygen demand (COD). The devastation of aquatic life and other marine organisms due to the discharge of dairy plant effluents into water resources can further deplete oxygen levels and provide extra food for microbial consortia. Anaerobic systems have been developed due to their ability to use methane gas and treat wastes with great strength [2]. One method of continuously operating anaerobic wastewater treatment is the Upflow Anaerobic Sludge Blanket (UASB). In the UASB, the wastewater flows vertically up from the bottom above the sludge blanket, where the waste is being digested by bacteria. Hydraulic retention time (HRT), up-flow velocity (Vup), and organic loading rate (OLR), changing operating temperature, and pH all had an important effect on UASB performance [3]. Low capital cost, low energy requirements, low operation and maintenance expenses, and sustainability are the fundamental criteria used when choosing technology for sewage treatment [4]. The gas collection, gas separation, feed input, and effluent withdrawal are the main physical components of a UASB reactor that need to be carefully considered. The UASB reactor is the only one with feed input and gas separation designs. In order to prevent channeling or the creation of dead zones, the feed inlet is designed to distribute wastewater uniformly [5]. The UASB method is a popular and reasonably priced technique for treating industrial wastewater. Anaerobic bacteria are used in this biological treatment process to break down organic materials in wastewater, with the end result being biogas [6].

Anaerobic digestion provides the dairy industry with a special treatment option as a result. Anaerobic digestion not only lowers an effluent's COD but also produces minimal microbial biomass. The largest benefit is the recovery of energy in the form of methane, and biogas can be produced from up to 95% of the organic matter in a waste stream. A common anaerobic reactor at both high and low temperatures is the UASB reactor. The most popular high-rate anaerobic method for treating anaerobic sewage is the UASB reactor. The most crucial factor influencing the size and form of the UASB reactor is the HRT rather than the OLR in the case of relatively weak wastewater, like sewage [7].

Several developments that have been reported for UASB include a 6-hour HRT experiment conducted to contrast the performance of a one-stage UASB reactor with a UASB digester system at low temperatures, such as 15 ℃. Researcher [8] discovered that UASB digesters outperform one-stage reactors in terms of conversion and separation efficiency. The sludge generated in the UASB reactor is considerably more stable. The UASB typically performs well in tropical nations. UASB was one of the potential methods for sludge stabilization and wastewater treatment. In order to handle wastewater in tropical regions, the UASB reactor was investigated at two distinct lower temperatures in this study.

Wastewater enters the UASB reactor at a natural temperature of between 22 and 26 ℃. HRT during the colder months is 2, 4, 6, 8, and 10 h with varying loading rates ranging from 0.95 to 5.70 kg COD/m³∙d. The removal effectiveness of BOD₅, COD, and TSS was 54, 46, and 53% during the colder period and around 71, 63, and 65% during the warmer period [9].

The investigated catechol biodegradation in aqueous solution by co-metabolism with glucose as the major substrate in a UASB reactor; according to their batch investigations, catechol in an aqueous solution may be co-metabolized and degraded up to a concentration of 1000 mg/L glucose at a concentration of 1000 mg/L. With a good COD removal efficiency of almost 95% [10].

Despite numerous studies addressing dairy water treatment by UASB, these studies have not focused on evaluating reactor performance using key operating conditions, namely, flow rate (Q), HRT, and environmental conditions (temperature, pH) during the summer season. Real operational data from a UASB reactor that treats dairy wastewater in Al-Samawa, Iraq, over the summer is presented in this article. The work is important since it evaluates the impacts of Q, HRT, temperature, and pH at high ambient temperatures, as well as whether the treated effluent is suitable for environmental discharge regulations. The results offer a regional case study and useful recommendations for the use of inexpensive anaerobic treatment systems in the Iraqi dairy sector.

The main objective of this research is to evaluate UASB technology as an alternative and efficient method for treating dairy wastewater in Al Samawa City under different operating conditions. It also aims to determine the HRT that provides the best removal rates of COD, BOD, TN, and TSS, and to compare the UASB effluent with effluent discharge standards.

During this research, three discharges (11.6, 5.8, 2.91) L/h were adopted, with HRT in the reactor ranging from 6, 12, and 24 h.

The equation that follows is used to calculate the removal efficiency of any pollutant [12]:

Removal efficiency $\%=\frac{\left[A_o-A_t\right]}{A_o}$           (1)

where,

A_o = Pollutant concentration at time zero, mg/L

A_t = Pollutant concentration after time t, mg/L

The following formula was applied to calculate the OLR [13]:

$O L R=\frac{\operatorname{COD}_{i n}}{H R T}$           (2)

where,

COD_in = Influent COD concentration (kg/m³)

HRT = Hydraulic retention time (day)

Table 4. Experimental operation

At a temperature of 35–37 ℃, the DO values were 0.31–0.01 mg/L in dairy wastewater. The rates of pH varied; it recorded the lowest value of 7.2 at HRT 6 h and the highest value of 7.8 at HRT 24 h. The rates of EC varied; it recorded the highest value of 3634 mg/L at HRT 6 h and the lowest value of 3103 mg/L at HRT 24 h. The rates of TDS varied; it recorded the highest value of 2543 mg/L at HRT 6 h and the lowest value of 2117 mg/L at HRT 12 h. Table 4 shows the experimental operation.

3.1 Chemical oxygen demand removal at different hydraulic retention times

Figure 2 shows monthly changes in the summer months (July, August, September) of COD results at different HRTs. In Figure 3, the COD removal is over 95%. At HRT 6 h, the effluent COD concentrations diminished from 435 mg/L to 293.2 mg/L. The corresponding OLR, determined using the operating conditions, decreased overall from 7.28 to 6.79 kg COD/m³∙d, while the average removal efficiency increased from 76.09% to 82.73%.

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At HRT 12 h, the effluent COD concentrations diminished from 379 mg/L to 290.3 mg/L with OLR from 3.6 to 3.3 kg COD/m³∙d, while the average removal efficiency increased from 78.97% to 83.37%. At HRT 24 h, the effluent COD concentrations diminished from 214.6 mg/L to 72 mg/L with OLR from 1.8 to 1.6 kg COD/m³∙d, while the average removal efficiency increased from 88.27% to 95.56%.

A comparison between UASB effluent quality and discharge standards showed that, at an HRT of 24 h, the COD values in Tests 8–12 complied with the Iraqi wastewater discharge specifications for WWTP effluents. COD at 24 h in all tests (1–12) complies with Environmental Health and Safety in Table 5. Table 6 shows contaminant compliance with standard limits by HRT and test number. Table 7 shows the results of the COD concentration.

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Table 5. The effluent standards of certain pollutants in countries

Table 6. Contaminant compliance with standards

Table 7. Results of chemical oxygen demand (COD)

3.2 Biochemical oxygen demand removal at different hydraulic retention times

Figures 4 and 5 show the BOD results and removal efficiency at different HRTs, respectively. At HRT 6 h, the effluent BOD concentrations diminished from 296 mg/L to 205.8 mg/L, while the removal efficiency increased from 60.53% to 70.6% (Table 8). At HRT 12 h, the effluent BOD concentrations diminished from 207.3 mg/L to 148 mg/L, while the removal efficiency increased from 71.60% to 77.57%. At HRT 24 h, the effluent BOD concentrations diminished from 101.6 mg/L to 39.3 mg/L, while the removal efficiency increased from 85.94% to 94.10%. A comparison of UASB effluent quality and discharge standards showed BOD at 24 h in only Tests 11 and 12, which could be compared with Iraqi Specifications from WWTPs and Environmental, Health, and Safety (Table 5).

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Table 8. Results of biochemical oxygen demand (BOD)

3.3 Total nitrogen removal at different hydraulic retention times

Figures 6 and 7 show the TN results and removal efficiency at different HRTs, respectively. At HRT 6 h, the effluent TN concentrations changed slightly from 55.0 to 55.3 mg/L, while the removal efficiency increased from 16.66% to 19.85% (Table 9). At HRT 12 h, the effluent TN concentrations diminished from 44.8 mg/L to 40.5 mg/L, while the removal efficiency increased from 18.54% to 20.43%. At HRT 24 h, the effluent TN concentrations reduced from 54.1 mg/L to 45 mg/L, while the removal efficiency increased from 20.90% to 25.74%. A comparison of UASB effluent quality and discharge standards showed that TN does not meet all standards. The anaerobic nature of the UASB system and the absence of integrated nitrification and denitrification conditions hampered UASB's ability to remove TN. As a result, TN concentrations continued to be higher than those allowed by environmental regulations.

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Table 9. Results of total nitrogen (TN)

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3.4 Total suspended solids removal at different hydraulic retention times

Figures 8 and 9 show the TSS results and removal efficiency at different HRTs, respectively. At HRT 6 h, the effluent TSS concentrations diminished from 288 mg/L to 224 mg/L (Table 10), while the removal efficiency increased from 58.85% to 69.85%. At HRT 12 h, the effluent TSS concentrations diminished from 280 mg/L to 180mg/L, while the removal efficiency increased from 60.17% to 76.31%. At HRT 24 h, the effluent TSS concentrations diminished from 210 mg/L to 52 mg/L, while the average removal efficiency increased from 70.42% to 92.82%. A comparison of UASB effluent quality and discharge standards showed that TSS at 24 h in only Tests 11 and 12 could meet Iraqi Specifications from WWTPs and Environmental, Health, and Safety. As detailed in Table 5.

Table 10. Results of total suspended solids (TSS)

3.5 Description of operating conditions

Increasing the HRT resulted in lower effluent COD concentrations. An HRT of 24 h achieved the highest removal efficiencies: 95.56% for COD, 94.10% for BOD, and 92.82% for TSS. This is consistent with Utami et al. [3], a higher HRT usually results in greater reactor performance in terms of removal efficiency and has been observed to do so for an UASB with COD levels that HRT increased from 36 h to 72 h, increase in COD removal efficiency of 55.64% to 66.81%, BOD₅ from 67.85% to 74.58%; and TSS from 66.69% to 84.19% [3].

The results showed that the temperature range from 35 to 37 ℃ leads to increased removal of organic pollutants, COD, BOD, and TSS, as anaerobic bacteria become more active at high temperatures in July, August, and September.

Padilla-Gasca and López-López [16] found that the removal efficiency of COD was 86.1% at 35 ℃. The pH of the effluent from the UASB is between 7.2 and 7.8, and the COD and BOD removal increase with varying pH between 7.2 and 7.8. At pH 7.2, the COD removal is 76.098%, and at pH 7.8, the COD removal is 94.639%. More than 80% of COD is removed in UASB, which occurs best in an environment at a pH of 7 [17]. The pH range 6.5 to 7.8 in order to achieve greater results [18].

3.6 Comparing with other Upflow Anaerobic Sludge Blanket system studies

Table 11 shows that the removal efficiency of the present study was compared with the removal efficiencies of other system studies. This study's COD removal efficiency is higher than that of research by Tandukar et al. [19] and Seghezzo et al. [20], and is consistent with Elmitwalli et al. [21]. The BOD removal efficiency was the closest to that of research by Seghezzo et al. [20] and higher than that of Tandukar et al. [19].

Table 11. Comparison of the Upflow Anaerobic Sludge Blanket (UASB) with other studies