Development of Malaysian Wastewater Polishing Index: Case Study on a Moving Bed Biofilm Reactor

Currently, extra treatment of secondary effluent to remove nitrogen and phosphorous may be required for its unrestricted reuse. This can be achieved by installing the wastewater polishing systems (or tertiary treatment). The wastewater polishing solutions are environmentally friendly, cheap and effective. The experiments were conducted on a pilot scale using a Moving Bed Biofilm Reactor (MBBR) with a capacity of 500 L to polish the municipal effluent from organic pollutant, undesirable nutrients and bacteria without the use of disinfectants. The major purpose was to define and apply a model for evaluating polishing of secondary effluent and implement the optimal technology for unrestricted use. Wastewater Polishing Index (WWPI) is a new tool that has been employed for rapidly evaluating of water quality improvement. It can be implemented into any surface water effluent treatment system or for reuse. WWPI can be an important tool designed for decision makers. The total average weight of six parameters (COD, BOD5, SS, Ammonia nitrogen, Total Phosphorus and Escherichia coli) are defined as the index, each one converted to sub-index which is scaled from zero to one hundred. If none of six parameters exist in the effluent WWPI, it is equal to zero, while if the parameters match the Malaysian’s effluent Standard B, It amounts to one hundred. In turn, WWPI is ninety when all six of them are equal to their corresponding Malaysian National Water Quality Standard (IV) for re-use. The index of wastewater polishing was validated and approved for the pilot MBBR study.


INTRODUCTION
There are no common indices specified for monitoring regulatory standards or for quick evaluation of the obtained results during the wastewater recovery in Malaysia, only water quality index (WQI). Discharge of wastewater to surface water body or to any other final destinations should respect all the regulations set by local, national and regional quality standards (Morales- Garcia et al. 2011). In almost all countries, the trend towards concentration limits for the purpose of water reuse in several qualitative parameters was established, even without determining the more suitable treatment.
The objective of a wastewater polishing index (WWPI) is a rapid evaluation whether the polished effluent is suitable for agriculture or has the recreational potential. It has great benefits according to the managers and the people making the water planning decisions, also to compare different wastewater treatment technologies. WWPI is known as the average weighted of the following six parameters: COD, BOD 5 , SS, Ammonia nitrogen, Total Phosphorus, and Escherichia coli (Bhavin 2012). Each of these parameters was converted to a sub-index scaled from zero to a hundred.. If none of six parameters exist in the effluent WWPI, it is equal to zero, while if the parameters match the Malaysian's effluent standard B, it amounts to one hundred. However, in a number of cases, WWPI will exceed 100 because of high concentrations of suspended solids and E. coli within the secondary discharge. The value of WWPI is 36 in Italy where each of the six parameters is equivalent to the Italy legal restrictions on reuse (Bhavin 2012).
The increasing coverage of the domestic water supply resulted in a substantial increase in the wastewater production and combined with natural scarcity of water, increased the amount of wastewater to be treated (Al-Baldawi et al. 2013). The heavy usage of underground aquifers and continuous drop in, especially in dry regions, which has encouraged the reuse of domestic wastewater as substitute sources of water and tools of polishing (Palese et al. 2009 (Lariyah et al. 2016). It is used to remove turbidity, particulars, microorganisms as well as cysts (Oron et al. 2008) without any disinfectant usage in order to build up sustainable water supply. The advantages of MBBR are very promising by combining an integrated system (Bick et al. 2009;Drioli et al. 2011). The investigation focused primarily on the system of polishing using MBBR, as it is widely adopted worldwide for large, medium and small WWTP effluent (Tang et al. 2017). The MBBR technology is considered the most suitable biological and physical treatment for producing high effluent quality and consistency, which is sufficient for its reuse (Piechna & Żubrowska-Sudoł 2017; Lin 2018).
The pilot plant has been made on the treatment plant of the UKM Bangi engineering building, located in the city of Kajang, Malaysia, to evaluate the efficiency of MBBR system for secondary effluent polishing. The feeding water was taken straight from the WWTP secondary effluent of urban municipal clarifier. The three objectives of the current study were as follows: (i) to offer rapid tool evaluation for improving the water quality; ii) provide sufficient technical information and aid in understanding the performance of secondary supplementary effluent polishing using MBBR; iii) provide a reliable and sustainable recovered effluent, suitable for agricultural irrigation and reuse without restrictions.

Malaysian Treated Effluent Discharge Standards
In Malaysia, the standards of wastewater are provided in the environmental quality regulations of 2009 (sewage and industrial effluents). These standards cover not only the industrial waste water, but also apply the limit values to the domestic wastewater. The standards on wastewater are prescribed as a series of national standard uniform which is divided into two classes: the standard A is applicable to the areas upstream of the drinking water intake points, while the standard B is applicable to the downstream areas of the drinking water intake points. Every standard covered 23 parameters, including common parameters, for instance BOD 5 , COD, SS, pH, temperature, pH, CFU/100 ml and various types of heavy metals. The standard A is stricter than the standard B (Environmental Quality 2009).
The water quality standards (WQI) in Malaysia were set for the quality of the river waters. The water quality is divided to six categories (Department of Environment 2010), from the level where it retains the natural environment in which aquatic organisms are sensitive to environmental changes, through the level that the water can be used for drinking after tertiary treatment, to the level usable for irrigation in agriculture. The water quality standards are determined for about seventy parameters, including the ammonia nitrogen, COD, BOD 5 and bacteria coliform number of groups, as well as a large number of pesticide and heavy metals. There is no specified environmental standard for ponds and lakes, but an interim standard is currently proposed to be applied for coastal waters.

Development of Wastewater Polishing Index WWPI
Wastewater is polished in order to further improve the water quality via decreasing the concentrations of the essential parameters of pollution, until the level needed for their destination is reached (released into the body of surface water, reuse or recycling). The water quality improvement depends on the selected treatment of various polishing technologies and evaluates their purification performance. A modern indicator named wastewater polishing index has been invented. The main parameters are the very important toward the effluent into final destination or reuse of the recovered municipal wastewater. The parameters are COD, BOD 5 , SS, NH 3 -N, P Total and E. coli (Asano 1998;Metcalf & Eddy 2014).
Generally, the influent flow to the polishing stage is a secondary discharge, the quality is illustrated via the variety ranges of subsequent main parameters: BOD 5 10-20 mg/l, SS 10-30 mg/l, COD 30-60 mg/l, P Total 0.8-1.5 mg/l, NH 4 5-10 mg/l and E. coli 103-105 CFU/100 ml (Metcalf & Eddy 2014). Since these ranges are expressed with varied units and different sizes, a normalization step is needed in order for the corresponding sub-indices to be used in the same interval scale. The rating curve is recruited by two key points for all indicators matching the estimated limits for each specific scope (Table 1).
For the six parameters, the normalizing curve assumed a linear graph between the two extreme points, as illustrated in Figure 1. In this way, the matching of six sub-indices was identified. If the value of analyzed concentrations is greater than the value of the concentrations allowed by law, the corresponding linear correlation will over range which a normalized sub index value higher than 100.
The Eq. (1) defined the wastewater polishing index, where Ii is sub-index matching the essential parameters i, whereas i of COD, BOD 5 , SS, P Total , NH 3 N and E. coli is to 1 for all parameters, except for E. coli, which is equivalent to 1.4. Instead of the sub-index value, the WWPI will be calculated by the following equation: (1) where: WWPI is wastewater polishing index, I BOD5 is normalized value sub-index of BOD 5 , I COD is normalized value sub-index of COD, I NH3N is normalized value sub-index of NH 3 N, I P is normalized value sub-index of phosphorous, I SS is normalized value sub-index of SS, The highest value provided to the peak E. coli sub index to improve the experimental ability of polishing disinfection. WWPI shows that a macroscopic achieved quality of effluent is lower than the level permitted by Malaysian regulations for discharge to surface water body, where the WWPI value is 100. If the effluent quality meets the specific legal requirements, the recovered wastewater can be reused for industrial, agricultural or civil purposes. The Malaysian regulations were introduced by [Department of Environment 2010], referring to 23 parameters. However, the main problem for the domestic wastewater reuse usually occurs because of the high concentration of six parameters that have been selected to determine WWPI. These requirements are listed in column IV of Table 2.
If the six key indicators approximate appropriate national standards for water quality Malaysia (IV) limits for the reuse of recovered wastewater, the index is 90.

EXPERIMENTAL VALIDATION BASED ON TREATED EFFLUENT FROM MBBR
The pilot plant moving bed biofilm reactor MBBR was experimentally investigated in the WWTP, at a new engineering building of UKM for validating the new indicator. The inlet water (tertiary effluent) was taken immediately from the secondary clarifier effluent of the municipal WWTP. The experimental investigation In this pilot-scale study, experiments were conducted by designing, building, and operating a 500 L capacity moving bed biofilm reactor (MBBR) to polish the organic pollutant, undesirable nutrients and bacteria in the treated wastewater without the use of disinfectants. The 500 L MBBR includes a submerged clarifier designed with height and diameter dimensions of 120 and 75 cm, respectively, as shown in Figure 2. It has a down flow configuration and it is supplied with aeration from the bottom. The reactor was filled with 1250 pieces of fabricated Enviro Multi Media which is equal to 5% (v/v) of reactor volume.
All the samples of water were gathered on the same day and time in clean plastic bottles. The samples were directly analyzed chemically and physically in civil and environmental engineering laboratory. The analyses of essential parameters such as COD, BOD 5 , NH 3 N and P Total were conducted according to the standard American method for wastewater and water analysis (APHA/AWWA/WPCF, 2005). Suspended solids were measured using the DR6000 HACH spectrophotometer. The membrane filtration method was used to carry out E. coli counting at the incubation temperature 37 ± 0.5 °C for 24 hours. The WWPI equation takes E. coli into consideration, because the enteric bacteria removal is higher than the percentage removal for E. coli, as shown in the experimental investigation (Vymazal 2005;Salgot et al. 2006).

RESULTS AND DISCUSSIONS
The methodology used to determine WWPI is based on the normalized graph of parameter values (Bhavin 2012). WWPI depends on a single result for each parameter and the result shows less variation in the index. Different WQI depends on a great number of replicated samples analyzed over long-term. WWPI is quicker, less labor-intensive and less time-consuming process, whereas WQI takes a long time to determine. WWPI is used for wastewater treatment, while the WQI is used only for the drinking water. Table 3 shows in detail the averages and standard deviations of COD, BOD 5 , P Total , SS, NH 3 N and E. coli for effluent and influent of the pilot plant during experiments, where the MBBR technology produces extra high quality water.
Commonly, WWPI for influent is always below the threshold value of 100, but in a few cases, it may be higher as a result of E. coli and SS high concentrations in secondary discharge (Verlicchi et al. 2011). Accordingly, a chemical disinfection must be added to ensure meeting the Malaysian legal limit of 5,000 CFU/100 mL for discharge into surface water bodies. In the current study on effluent polishing using MBBR, the chemical disinfection was not used due to low concentrations of E. coli.
No sludge was discharged from MBBR during all the experimental period. . The MBB reactor with HRT 24hrs enhances the final effluent quality, where WWPI always corresponds to 2 under or close to class I. Figure 3 confirms that a moving bed biofilm reactor is normally enough for producing the effluent corresponding to class (I) that WWPI is equal to 3 and sufficient for direct reuse. In turn, a study was conducted in Italy for comparison of the effluent water quality index. They used wastewater polishing index (WWPI) in different refining treatments including a rapid sand filter, horizontal subsurface flow system, lagoon and their combinations. The WWPI results were between 10.25 and 40.14 (Bhavin 2012).
The Malaysian WWPI indicates macroscopically to what extent is the effluent quality achieved below the level permitted by Malaysian regulations for discharge to surface water body, where Furthermore, WWPI is defined as a flexible indicator similar to the model in equation (1). It can be expanded to further chemical parameters or to further biological parameters for specified demands. As an example, if standard restrictions specify other bacteria, virus, or protozoa, the corresponding weight to the every new parameter must be determined (in equation 1 in is the exponent). In addition, the index can be utilized for any country. Every country has various legal restrictions established for wastewater treatment plants effluent discharge into surface water or reuse. Thus, the wastewater polishing index would lead to new thresholds.

CONCLUSION
The most critical pollutants in secondary effluent of domestic treatment plants are: COD, BOD 5 , SS, P Total , NH 3 N, and first of all the E. coli bacteria. The water quality index (WQI) depends on a large amount of water samples analysed in the long term. Therefore, WQI takes a long time to identify and can be used only for drinking water. On the contrary, Wastewater Polishing Index (WWPI) is a new index proposed for an effluent polishing to give quick and more accurate results of the water quality level. The technique is based on a chart, thus simple to understand and easy to implement. The wastewater polishing  index can be a great support for those who make decisions and environmental management in the water resources planning. Therefore, WWPI could help them to make a quick assessment for a number of scenarios including different polishing treatment systems when necessary. MBBR with HRT of 24 hours can improve the effluent quality where WWPI corresponds with class (I) and can be employed for direct reuse. No sludge was generated during the experiment. Model simulation results were verified and approved. This model can be used for the design of a pioneer MBBR plant for simultaneous removal of organic carbon and nutrient from a wastewater treatment plant (WWTP).