Performance evaluation of package plant for treatment of single household domestic wastewater

The conventional septic tank has been in application since a long time as a popular mode of onsite domestic wastewater treatment in the absence of centralized sewerage treatment facilities. Despite its wide application, it has several drawbacks, including lower removal of biochemical oxygen demand (BOD) and total suspended solids (TSS) at around 30% and 50%, respectively (Coelho et al. 2003; Von Sperling & Chernicharo 2005). Disposal of partially-treated wastewater by septic tank makes a negative impact on surrounding aquatic environment, public health and groundwater quality.

Several alternatives have been developed for improving the effluent quality of onsite wastewater treatment systems, such as UASB-septic tank, membrane bioreactors, anaerobic baffled reactor and constructed wetlands (Nakajima et al. 1999; Abegglen et al. 2008). However, all these systems require large-sized reactors and high cost of construction.

Recently, there has been an increased application of package systems for the treatment of household wastewater all over the world due to several beneficial aspects than the conventional sewerage systems (Greaves et al. 1990). Therefore, this study focussed on performance evaluation of advanced package septic systems for onsite treatment of domestic wastewater as an alternative to the conventional septic tank. The effluent concentrations of organic matter and microbial pollutants of anaerobically-treated wastewater usually exceed the maximum permissible level prescribed by the effluent discharge standards of most of the developing countries (Tyagi et al. 2009). Therefore, to overcome this problem, post treatment of anaerobic effluent is necessary to reduce these parameters to the required level. Available data indicate that on the basis of the main water-quality parameters, electrochemically-treated wastewater can be successfully reused for specific applications, where a high degree of water quality is not a necessary requirement (Lin et al. 2005; Vakil et al. 2014).

Therefore, the present study focussed on characterization of raw domestic wastewater and its treatment using a package plant in combination with physico-chemical treatment. In addition to performance evaluation, detailed septage and sludge characterization was also carried out using a scanning electron microscope (SEM).

The package plant was made of linear low density polyethylene material. It consisted of two chambers of bioreactors: a settling tank followed by an upflow anaerobic filter. The second chamber was the core unit of the system and packed with pall ring media (polypropylene), which was designed to offer minimum surface area of 100 m² m⁻³ of packing. Total effective volume of the system was 1,200 L with the septic volume being 950 L and the media chamber volume as 380 L. The reactor was started without using inoculums and fed directly with the original domestic wastewater.

Removal efficiency of chemical oxygen demand (COD) and microbial content was assessed using a stirred tank electro-chemical reactor where different variations and arrangements of aluminium electrodes and combinations of aluminium and graphite electrodes were used. The bench-scale reactor used for the laboratory study was made of acrylic sheets. The effective dimensions of the reactor were 50 cm (H) × 20 cm (L) × 10 cm (W), having volume of 10 L. Four aluminium plates, each of effective size 48 cm × 8 cm × 0.2 cm, were used as electrodes. The electrodes were connected to DC power source in such a way that they acted as monopolar electrodes, either cathode or anode. The current and voltage through the system were measured using Multimeter. Spacing between the electrodes was 4 cm.

The detailed characteristics of raw domestic wastewater during the study are summarized in Table 2. The average ratio of COD: BOD for the domestic sewage was observed to be 2.31 ± 0.55, which was a good indication that the domestic sewage could be successfully treated by biological treatment (Metcalf & Eddy 2003).

Wastewater samples were collected weekly from influent and effluent of the system at the time of peak flow hours. All the collected wastewater samples were analysed for pH, COD, BOD, total organic carbon (TOC), TSS, total nitrogen (TN), total phosphate (TP), total coliform (TC) and fecal coliform (FC). All the analyses were performed according to Standard Methods (APHA 2005).

Septage samples were also collected from the primary chamber and analysed for the above-mentioned parameters on a monthly basis.

Sample preparation for SEM analysis included fixation of sludge sample for 1 h at 4.4 °C with 2.5% (w/v) glutaraldehyde in phosphate buffer solution and dehydrated through series of acetone in distilled water solution. These samples, in each mixture, were then brought to equilibrium for 10 minutes and finally dried off by the critical-point drying method before being sputter-coated with gold particles (Sharma et al. 2014). Finally, the samples were examined using SEM (LEO 435 VP).

The average amount of generated domestic sewage over the period of summer was found to be 140 L per person per day. This corresponds to an average water consumption rate varying from 150 to 190 L per person per day for households living in the apartments that includes some quantity of water used for irrigation or landscaping also (Metcalf & Eddy 2003).

Out of the total wastewater generated from different consuming points, kitchen contributed the most in terms of domestic sewage (about 42%) and COD loading (about 37%) on the system. However, black water generated from toilet flushing was found to be only about 18% of the total wastewater generated as well as COD loading. The distribution of domestic sewage generation and its relative COD loading from the different consuming points are shown in Table 1.

The package plant was started in November 2012 without using inoculums and operated for a period of 530 days (approximately one and half year). The performance of the system was regularly monitored throughout the study period. The average performance characteristics of the system during the study period are summarized in Table 2.

The average effluent pH and alkalinity were observed as 7.36 ± 0.32 mg/L and 351 ± 38 mg/L as CaCO_3, respectively, showing the buffering capacity of the anaerobic reactor.

The average effluent concentrations of COD and TOC were observed to be 208 mg/L and 98 mg/L with 70.9 ± 11.8% and 62.1 ± 5.9% removal efficiency, respectively. It was clear from the data that the percentage removal of TOC was lower than the COD, which might have happened because the analysis of COD includes measurement of several compounds like metallic cations, inorganic compounds etc. that are not included in the TOC measurements. In addition, some of these compounds have the tendency to be absorbed by the biofilm, which results in an increase in the percentage removal and effluent concentration of the COD (Perez et al. 2007).

Removal of nutrients from the domestic sewage is not quite satisfactory in anaerobic treatment systems (Mohapatra et al. 2012). During the present study, the system displayed 13.8± 3.7% removal efficiency for the TP with the effluent concentration of 8.2 ± 2.3 mg/L. The removal of phosphorous might be attributed to its consumption for biomass growth, precipitation and entrapment within the digested sludge (Wanasen 2003). Similarly, the system removed a very small portion of the TN, probably due to ammonia volatilization. The average removal efficiency for the TN was observed as 20.2 ± 8.4% during the study period.

On an average, the log₁₀ reductions of TC and FC were quite satisfactory, which were quantified as 1.30 log (89.9%) and 1.10 (86.5%), respectively. This might be attributed to the combined effect of physico-chemical process coupled with natural die off, and presence of toxicity of the specific pathogens (Yang et al. 2000).

Septage is another important operational parameter of anaerobic onsite systems, which includes the sludge accumulated at the bottom of the primary chamber and the scum that floats to the surface of the liquid layer. Average characteristics of the septage during the study period are illustrated in Table 3. It can be seen from the table that average concentrations of all the parameters were within the range as reported by the USEPA (2002) for domestic wastewater treatment.

SEM morphology revealed presence of various bacterial species of methanococcus and methanosaeta and inert material in the flocs. Two types of cocci were observed within the reactor: large and small. These cocci resembled species of the methanococcus genus, which plays an important role in methanogenesis, the final stage of anaerobic digestion process (Figure 4(a) and 4(b)). The other morphotypes, as illustrated in Figure 4(a), were typically characteristic of the acetoclastic methanogen and methanosaeta.

Figure 5(b) shows the percentage removal of FC from the domestic wastewater over a period of time at different voltages in the electrochemical cell. It was observed that the increase in the voltage improved the removal of the pathogenic indicators. The figure shows that within an hour almost 97–99.9% of the FC were removed from the anaerobically-treated domestic wastewater. This removal efficiency revealed that the electrocoagulation process was highly significant in coliform reduction. Additionally, the average concentration of FC was observed as 2.2 × 10³ ± 1.8 × 10³ MPN/100 mL in final treated effluent of electrochemical reactor that fulfilled the permissible limit of 1,000 MPN/100 mL prescribed by WHO (1989) for unrestricted irrigation.

Onsite treatment systems are considered as the most cost-effective option of domestic wastewater management in unsewered areas in comparison to centralized sewerage treatment system (USEPA 1997). For effective wastewater management, the treatment system should be environmentally, socially and economically sustainable (Engin & Demir 2006). However, in actual practice, the selection of treatment system mainly depends upon the cost involvement, including construction demands, land requirement and operation and maintenance expenses. Due to low cost involvement, conventional septic system is widely used in developing countries. Therefore, it was important to compare the cost of the present treatment plant with the conventional system. Table 4 provides the cost analysis of the present treatment system, including the cost of electro-chemical treatment.

The results of the present study revealed that the package-type settling-anaerobic filter system delivered appreciably higher removal efficiency for BOD and TSS, which was quantified as 68.7 ± 8.5% and 78.1 ± 4.7%, respectively. The removal efficiency for BOD and TSS in the conventional septic tanks with similar volume varies between 30–40% only. Therefore, on the basis of much higher pollutant removal efficiency than the CST, the present system can be a feasible alternative to the conventional septic tank.

The package system produced lower pollutant concentrations in the effluent with the average values of main water-quality parameters of BOD and TSS observed to be 123 ± 51 mg/L and 85 ± 23 mg/L, respectively. Although, the system provided a better effluent than the CST, it still contained high pollutant concentrations, usually exceeding the maximum permissible levels prescribed by the effluent discharge standards of most of the developing countries, including India. To overcome this hurdle, electrocoagulation can be a promising technology for the post treatment of anaerobically-treated effluent. In this study, the bench-scale electrochemical process using aluminum electrodes proved successful in significantly removing the pollutants in terms of the COD content. The electrocoagulation process facilitated further increase in the COD removal efficiency, up to about 70%.

The authors are thankful to the Ministry of Drinking Water Supply and Sanitation, Government of India, New Delhi, India for providing financial support for the study (project number MRD-553-CED).