Application of integrated MBR in a centralized WTP for a chemical industrial park: A case study in China

Wastewaters from chemical industries usually contain organic and inorganic matters in varying degrees of concentration. Many of them are toxic, carcinogenic, mutagenic and non-biodegradable. As such, treatment of chemical wastewaters is always a challenging topic in view of the strict environmental regulations. Since 2014, the Chinese government has been tightening the industrial wastewater (IWW) discharge standards, which has led industries to continuously improve the quality of their IWW effluent. This poses great challenges to the chemical industries in China, especially to many of the chemical industry clusters where the wastewaters are usually more complex.

With the more and more stringent standards, large numbers of upgrading and retrofitting projects for industrial wastewater treatment plants (IWTPs) have been conducted in China, using different technologies and processes. Among many of the technologies available, membrane bioreactor (MBR) is regarded as a reliable and cost-effective solution for the treatment of IWW. However, MBR alone could not effectively remove non-biodegradable organics. As such, integrated MBR processes comprising MBR, advanced oxidization process (AOP) and/or other physical-chemical technologies are desired to improve the overall treatment efficiency of waters from various waste streams (Durán-Moreno et al. 2011).

Industrial Wastewater Treatment Plant-A (IWTP-A), located in Industry Park-W (Jiangsu province, China), is a centralized wastewater treatment plant responsible for treating both municipal and industrial wastewaters from the Industry Park-W. The normal daily influent of municipal wastewater is 5,000 m³/d, whilst daily IWW influent is 7,000 m³/d. The industrial waste streams mainly come from 15 companies in Industry Park-W, covering textile, chemical, pharmaceutical and pesticide manufacturing. CITIC Envirotech Ltd. (previously known as United Envirotech Ltd.), acquired IWTP-A in 2014 and upgraded the process to an MBR plant with design capacity of 20,000 m³/day. To further improve the treatment efficiency, different technologies are proposed to be coupled with the existing MBR, including Fenton + MBR, MBR + ozonation + biological aerated filter (BAF), and MBR + porous resin sorption (Schaar et al. 2010; Xu et al. 2003, 2014). In this paper, the performance of different integrated MBR processes will be discussed and evaluated on their efficiency in treating the wastewater from Industry Park-W.

The wastewater samples were collected from individual companies in the industrial park. Samples were stored at 4^◦C for characterization. SEPLITE Polymer resins (Sunresin, Shaaxi, China) with a diameter of 0.315 mm and density of 0.7–0.8 mg/ml were used for the resin adsorption testing.

A pilot-scale MBR system comprising a 24 m³ anoxic/aerated tank with submerged Memstar polyvinylidene difluoride hollow fibre membrane modules (Memstar Pte Ltd, Singapore) (membrane area: 50 m² per cartridge; pore size: 0.05 μm) was used in this study.

‘Standard Methods for the Examination of Water and Wastewater’ were adopted in this study for the measurement of chemical oxygen demand (COD), ammonia, and mixed liquor suspended solids (MLSS) (Clesceri et al. 1998). Total organic content (TOC) was measured by SHIMADZU On-line TOC-V_CSH (SHIMADZU Corporation, Kyoto, Japan).

All biodegradability tests followed the OECD 301-A protocol established by the Organization for Economic Co-operation and Development. The protocol contains measures of dissolved organic carbon (DOC) removal over a period of 28 days (OECD 1994).

Class 1(A) from China's Environmental Protection Agency (EPA) was adopted as the wastewater discharge standard in this study. Table 1 shows the major control parameters of Class 1(A).

The wastewater collected and treated by the IWTP could be divided into three streams, where the most difficult stream is chemical IWW. The chemical IWW samples were collected from 5 major chemical plants in Industry Park-W. Table 2 shows the characteristics of the respective wastewaters.

As indicated in Table 1, the BOD to COD ratios of the wastewaters were very low, suggesting a high hard-to-degrade COD composition in the wastewater. In order to predict the concentration of non-biodegradable organics, biodegradability tests were conducted with synthetic industrial chemical IWW composed of wastewater from companies A to E.

The biodegradability test is to evaluate the potential biodegradability of chemicals under optimized aerobic conditions. A biodegradability test was undertaken to give a good understanding of the ultimate biodegradability of the IWW after eventual bacterial acclimatization. Figure 1 shows the results from the biodegradability tests.

As shown in Figure 1, the initial COD concentration of synthetic chemical IWW was 350 mg/L. After 28 days of aerobic degradation, the ultimate COD reduced to and stabilized at around 180 mg/L. The total removal of COD in the aerobic degradation was thus about 48.5%, which suggests that degradation by microbial activity was limited at this level.

In this test, the Fenton process was applied prior to MBR as pre-treatment. The Fenton process is believed to be able to remove around 60% COD and improve the biodegradability of chemical compounds. The results of the pilot test using the Fenton process are show in Table 3.

Biodegradability tests were conducted on the effluents after the Fenton process. The initial COD of the raw wastewater was 180 mg/L (Figure 2). After 28 days, the ultimate COD was 50 mg/L. Thus, the removal of COD during aerobic degradation increased to 72%. Compared to the removal efficiency of pure biological treatment (48.5%), pre-treatment with the Fenton process significantly increased the biodegradability of the wastewater.

In this process, ozone was applied after the biodegradation process to further remove COD and convert non-biodegradable COD to BOD. The ozonation was then followed by a bio-aeration filter (BAF) to polish the permeate water. It was found that ozonation + BAF could remove 50% of COD in MBR permeate (Table 4).

Macroporous resins are able to adsorb organic pollutants due to their porous polymeric matrix. Many applications of macroporous resins are reported for effluent polishing in IWW treatment. Over 25 organic species could be effectively removed by these resins, including ketones, alcohols, benzenes, phenols, aniline, indenes, alkyl benzothiophenes, etc. Among all macroporous resins, SEPLITE XDA-1 hyper-cross linked polystyrene macroporous adsorbent is widely used for treating organic polluted water.

In this study, the pH of simulated IWW was adjusted to 4–5. The hydraulic retention time (HRT) of adsorption was 12 min. 30% sodium hydroxide was applied for desorption. After 10 adsorption-desorption cycles, the adsorption capacity was found to be well maintained.

Figure 3 shows the results of COD removal by macroporous resin adsorption. The results show that macroporous resin can effectively treat the synthetic IWW and reduce COD concentrations by 55%.

The results of this study suggest that Fenton's reagent and macroporous resin adsorption show good COD removal (from 37 to 60%) in the pre- and post-treatment of IWW from Industry Park-W. When considering the influence of pre-treatment processes on the biodegradability of raw wastewater, Fenton's reagent showed the best removal of COD (up to 65%) and the lowest ultimate COD after biological treatment.

Thus, a combined system including Fenton, oxidation and biological treatment (MBR) has been determined to be the optimal solution for process improvement of the IWTP-A. However, to ensure the effluent quality within the discharge standard of Class 1(A) from China's EPA, ozonation will be applied as post treatment to polish the MBR permeate. This combined treatment is proved to be an efficient and cost-effective approach to remove COD from IWW discharge.