Decentralized wastewater treatment using ‘Pumped Flow Biofilm Reactor’ (PFBR) technology

The protection and management of water resources is crucial for sustainable development and is therefore prioritized by governments and international organizations – e.g. UN and WHO. Both developed and developing economies focus on wastewater infrastructure investments. Challenges to the wastewater industry include climate change, population growth, a volatile global economy, increasing energy prices, heightened environmental awareness, and more complex regulatory and social circumstances (Marlow et al. 2013). To combat these, decentralization is increasingly recognized as an alternative management strategy to the norm of centralized wastewater collection and treatment. Decentralization is being adopted widely, e.g. in the USA, Japan, Australia and Italy (Libralato et al. 2012). Small-scale decentralized wastewater treatment systems, however, have design and operational challenges that can affect performance, including; (i) lack of permanent operators and local expertise, (ii) relatively high energy costs, (iii) sludge handling, (iv) compliance with strict discharge licenses, (v) variable influent hydraulic or organic loads, and (vi) inflexible operating regimes.

In Europe the Urban Wastewater Treatment Directive (UWWTD) (91/271/EEC) specifies standards for effluent discharged from places with population equivalents (PEs) exceeding 2,000. The Water Framework Directive (WFD) (2000/60/EC) specifies quality standards for effluent discharged from places with certificates of authorization (<500 PE) and with discharge licenses (>500 PE). These directives also regulate monitoring requirements, protect high-status waters and prevent further deterioration of all waters. Tight regulations challenge decentralized wastewater treatment plant (WWTP) operators to meet performance targets against an increasing need to minimize operating costs.

Approximately 80% of European WWTPs are designed for less than 5,000 PE (García 2009, EPA 2013). In Ireland approximately 94% of WWTPs operated by Irish Water have a PE below 10,000; 83% of these plants serve PEs below 2,000 (EPA 2014). Similar situations exist elsewhere. For example, 57% of the China's population live in 2.79 million villages accounting for 768.8 million people (Guo et al. 2014). Many small-scale WWTPs are likely to be unmanned.

Compliance with regulations varies widely. In Europe, average secondary treatment compliance rates are 88% for the original 15 EU member states but 39% in states that joined after 2004 (EC 2013). In Ireland, despite significant investment, 21% of WWTPs (>2,000 PE) have not complied with discharge standards (EPA 2014). Many WWTPs serving populations of less than 2,000 PE may have little or no monitoring, further exacerbating the situation. Furthermore, the ecological status of 58% of European surface waters is deemed less than good. In China, the quality of 40.1% of river waters is Grade IV or below (WEPA 2012). Despite recent urbanization in China, approximately 50% of discharges originate from rural areas, where increasing numbers of flush toilets contribute this challenge (Guo et al. 2014).

In addition to strict discharge limits, increased concern related to energy efficiency is a key focus in the wastewater sector (Kim & Hao 2001). The transport and treatment of water and wastewater is estimated to account for 7 or 8% of the world's energy consumption, with 4,600 Mm³ of water consumed annually to generate it (IEA 2012; Hoffman 2014). These figures are expected to rise by 33% by 2020 (IEA 2012), so methods of reducing energy consumption must be explored throughout wastewater treatment stages. In general, approximately 33% of the total operating cost of WWTP's is attributed to energy requirements (Fernández et al. 2011), with that needed for aeration representing up to 65% of such consumption (op. cit).

Increased difficulties also arise from sludge handling in small-scale WWTPs. WWTP sludge disposal in Europe is subject to increasing legal and social constraints, and accounts for up to 60% of plant operating costs (Paul et al. 2006). Reducing excess sludge production is appealing; particularly at decentralized, or smaller or remote facilities, where sludge treatment or re-use may not be feasible on-site, and transport costs can be high.

Operation and maintenance costs associated with small-scale WWTPs are of further concern (O'Reilly et al. 2011). Sophisticated wastewater treatment systems in remote areas can be hindered by lack of skilled workers, and operating and maintenance simplicity can determine the long-term success of a plant (Kalbar et al. 2012).

The pumped flow biofilm reactor (PFBR) is a novel, biofilm-based, batch process technology, designed to alleviate the issues cited above. Although it has been extensively tested at laboratory- and pilot- scale, and tested on-site, a full small-scale, municipal application has not yet been examined (Rodgers et al. 2004; Zhan et al. 2006; O'Reilly et al. 2008). The aim of this study is to investigate PFBR performance in treating municipal wastewater in a village in Ireland. The study was made in relation to effluent quality, energy consumption, sludge accumulation and maintenance requirements.

Sludge pumps were installed as a means of preventing sludge accumulation on the bottom of the reactor chambers. The clarifier was equipped with a submersible sludge pump, a level sensor and an electrically actuated discharge valve. The latter controlled the slow release of treated wastewater into the receiving water course, buffering the impact of batch releases from the PFBR. The balance tank was fitted with two submersible hydraulic feed pumps, one per PFBR stream, and a level sensor.

The PFBR was controlled by a programmable logic controller that allowed the operator to vary control parameters like water levels and treatment cycle length. All process information was displayed on a screen.

The study, conducted over five months, was split in two –Phase 1 (Days 28 to 36) and Phase 2 (Days 100 to 146). Initially the PFBR was operated to achieve carbonaceous oxidation and nitrification. On Day 37 the regime was enhanced to improve nitrogen removal efficiencies (Table 1). This was achieved by prolonging rest periods between reactor transfers and equalization times, and introducing an anoxic period, thus increasing the bacteria-wastewater contact time. Streams A and B were operated in the same manner, and received similar wastewater loads. The parameters analyzed included, organic carbon and nitrogen removal, sludge accumulation, hydraulic flows and energy consumption.

Throughout the study, daily composite influent and effluent samples were taken using refrigerated automatic samplers. Influent samples were taken from the balance tank and effluent samples from the clarifier chamber of Stream A prior to discharge. Influent flows were measured using an ultrasonic sensor and a flume (Siemens Hydroranger 200).

Total chemical oxygen demand (COD_t) and total suspended solid (TSS) were tested in accordance with standard methods (APHA AWWA & WEF 2005). Five day carbonaceous biochemical oxygen demand (cBOD₅) was measured using WTW Oxitop meters with a Lovibond nitrification inhibitor (N-ATH). Filtered and unfiltered total nitrogen (TN and TN_t, respectively) were measured using a Biotector TOC TN TP Analyser. Filtered ammoniacal-nitrogen (NH₄-N), and nitrite- and nitrate- nitrogen (NO₂-N and NO₃-N) were measured using a Thermo Clinical Labsystem, Konelab 20 Nutrient Analyser. Filtered wastewater samples were passed through 1.2 μm Whatman GF/C microfiber filters. Energy usage was measured with a built in Socomec energy meter, but only during the final two months of the study. Hach sc1000 multimeters monitored data collected from ammonium, nitrate and dissolved oxygen probes at 1 minute intervals. Each probe was maintained 300 mm off the floor of Reactor 1. All instruments were calibrated in accordance with manufacturers' instructions.

Submersible hydraulic pumps were installed in the voids at the base of the media modules to manage potential sludge build up. As there was no such build up, the pumps were never used during the 5-month study. When the study ended, a remote-controlled, closed circuit television (CCTV) survey was conducted to confirm the lack of sludge. The CCTV camera was put into the voids of both reactor, 1 and 2, of Stream A. It was used to examine all areas, especially the rectangular tank corners, where sludge was expected to build-up. The same study was repeated 13 months later, when the sludge pumps had been operated once each month, moving 5.3 m³ to a sludge storage tank that overflows into the primary tank.

During the start-up period (Days 1 to 27), effluent discharged from the PFBR system was recirculated to the primary settlement tank. In addition, the existing trickling filter system was operated to ensure that there was no additional risk to the receiving water during PFBR commissioning. During these four weeks a population of micro-organisms was allowed to develop naturally; no external seed sludge was used. After this time, the PFBR had reached a pseudo steady-state whereby average daily effluent results were consistent. Table 2 summarizes system performance during experimental phases 1 (Days 28 to 36) and 2 (91 to 146).

Influent and effluent oxygen demand were analyzed as BOD₅ and COD_t. The influent and effluent BOD₅ concentrations for Phases 1 and 2 averaged 151 mg.L⁻¹ and 8 mg.L⁻¹, and 121 mg.L⁻¹ and 3 mg.L⁻¹ respectively. This equates to average loading rates of 56 and 59 g-BOD₅.person⁻¹.day⁻¹ for phases 1 and 2 respectively. Average removal rates were 0.57 (Phase 1) and 0.47 g-BOD₅.m⁻²_{media surface area}.day⁻¹ (Phase 2). Influent and effluent COD_t concentrations for Phases 1 and 2 averaged 179 mg.L⁻¹ and 26 mg.L⁻¹, and 149 mg.L⁻¹ and 24 mg.L⁻¹, respectively, equating to removal rates of 0.61 g-COD.m⁻²_{media surface area}.day⁻¹ and 0.50 g-COD.m⁻²_{media surface area}.day⁻¹ for Phases 1 and 2, respectively. The facility consistently complied with the UWWTD discharge limits of 125 mg-COD_t.L⁻¹ and 25 mg-BOD₅.L⁻¹ throughout the study.

Influent TSS averaged 161 (Phase 1) and 54 mg-TSS.L⁻¹ (Phase 2). Effluent concentrations averaged 4.3 mg-TSS.L⁻¹ and 3.6 mg-TSS.L⁻¹ for Phases 1 and 2, respectively. These figures represent removal efficiencies of 97% (Phase 1) and 93% (Phase 2). All effluent concentrations were well within the UWWTD discharge limit of 35 mg-TSS.L⁻¹ (EPA 2012).

Energy use in Stream A was measured by component; (i) one 3.1 kW feed pump, (ii) two 3.1 kW circulation pumps, (iii) two 1.97 kW reactor chamber sludge pumps, (iv) one 1.97 kW clarifier chamber sludge pump, (v) four electrically actuated valves, and, (vi) control and monitoring equipment. The energy consumed by the domestic lighting and heaters, and the preliminary screening system and its controls, was not included in the analysis as they were subject to the control of the local governing authority operator. The use of light and heat can vary widely between WWTPs and depend on the operator on site. At small WWTPs these energy consumers (in particular heat) can impact significantly on energy use, due to operator preferences and/or heating being left on unintentionally. The preliminary screening system is an integral component of the overall system but its power consumption would have been minimal. Average energy requirements during the two-month test period were 0.22 kWh.m⁻³ treated and 1.74 kWh.kg BOD₅ removed⁻¹.

It is generally accepted that smaller facilities have higher unit costs because of lack of economies of scale. Typically the range for large conventional activated sludge plants is between 0.3 and 1.89 kWh.m⁻³ for inflows between 600 and 283,000 m³.day⁻¹ (Mizuta & Shimada 2010). However, despite the PFBR being relatively small and the impacts of storm water (which can cause operational challenges), its energy consumption averaged 0.22 kWh.m⁻³. Furthermore, the passive aeration technique employed in the study can be easily controlled and optimized, by adjusting the rest periods in each tank and varying the number of pumping cycles.

Where:

• X_e = effluent suspended solids concentration, and

• S_o and S_e = influent and effluent substrate concentrations, respectively

Excess sludge yields were estimated at 0.03 g-TSS.g-COD _removed⁻¹ for each of Phases 1 and 2. These sludge yields are low relative to other technologies; typically, activated sludge technologies, for example, have a sludge yield of 0.4 g-VSS.g-COD⁻¹ (Eddy & Metcalf 2004). While the sludge yields achieved are low, studies on the optimization of aerobic technologies treating paper pulp for reduced sludge production, have achieved 0.01 g-SS.g-COD _removed⁻¹ (Wei et al. 2003). Limited data are available on sludge yields for biofilm-based systems and the above figures should be taken as estimates only. The estimated sludge yield was favorable compared to that of a similar technology – the air suction flow biofilm reactor (ASF-BR) – which generated yields of between 0.2 and 0.69 g-SS.g-COD_{f removed}⁻¹, treating municipal wastewater (Clifford et al. 2013). In that study the biofilm growth medium comprised spherical, perforated balls. Clifford et al. (2013) hypothesize that the predominantly horizontal or slightly slanted surfaces were not conducive to biofilm attaching strongly to the medium and thus intermittent sloughing occurred, resulting in relatively high yields in the ASF-BR. In the PFBR, the biofilm is attached to vertically aligned media and thus resists gravitation forces and may also be less likely to slough as it adheres strongly (the operational characteristics of both the ASF-BR and PFBR were similar but the media were different, as mentioned above).

In this study, the biomass was exposed to repeated and prolonged relatively low organic carbon concentrations due to the nature of the influent wastewater that frequently included significant volumes of storm water. Under starved conditions, micro-organisms primarily utilize endogenous processes (Lu et al. 2007), which include cell maintenance and lysis, and regeneration, and can result in more substrate being respired to carbon dioxide and water, thus reducing biomass production (Low & Chase 1999; Abbassi et al. 2000; Liu & Tay 2001). Predation can also be an issue in biofilm systems (Parker et al. 1989) but it was not possible to measure the impacts (if any) of predation directly in this study. As the effluent quality remained high throughout this study, it is considered unlikely that there was significant biofilm loss by predation.

Repetition of the CCTV monitoring, after 13 months, showed a small build-up of sludge in both reactors of Stream A. The maximum depth of sludge accumulation was visually assessed, using CCTV, at 70 mm and 110 mm for Reactors 1 and 2, respectively. Sludge accumulations of 0.055 and 0.048 m³ for the two reactors were determined by visual estimation of the sludge surface areas in each reactor and applying Equation (1), used previously to estimate sludge yields in biofilm reactors (Rodgers & Clifford 2009).

As the facility only used hydraulic pumps and electrically actuated valves, maintenance was kept to a minimum. Pumps are usually maintained bi-annually or when required. Furthermore if pumping equipment fails it can be easily replaced by stand-by pumps. To reduce down time, every pump was fitted with a plug and socket electrical connection. This enabled the local caretaker to change a damaged pump single-handedly in an hour, without the assistance of an electrician.

A novel, municipal-scale wastewater treatment process has been designed, constructed and commissioned, and tested and analyzed over 5 months.

During Phase 1 of the study the average loading rates of BOD₅, TN_f and NH₄-N were 0.6 g.m⁻².day⁻¹, 0.065 g.m⁻².day⁻¹ and 0.038 g.m⁻².day⁻¹, respectively. The estimated sludge yield was 0.032 g-SS.g-BOD_{5 removed}⁻¹.

During Phase 2, the system was optimized for enhanced nitrification, and average loading rates of BOD₅, TN_f and NH_4­-N were 0.43 g.m⁻².day⁻¹, 0.07 g.m⁻².day⁻¹ and 0.05 g.m⁻².day⁻¹, respectively. The removal rates achieved during Phase 2 were 98%, 31%, and 94% for BOD₅, TN and NH₄-N, respectively. The estimated sludge yield was 0.03 g-BOD_5removed⁻¹.

The study showed that the installed system effectively achieved organic carbon and nitrogen removal. It also enabled excellent cycle control that can be used to optimize performance and minimize energy costs. The low sludge yields offer significant maintenance and financial benefits in decentralized environments. The system uses pumps only, and is robust and easy to maintain. The study shows the potential for passive aeration systems to meet new environmental and energy demands in the wastewater sector.

The PFBR is characterized by; (i) low operating costs; (ii) having no moving parts/compressors other than hydraulic pumps and actuated valves; (iii) producing little sludge, and (iv) being easy to operate and maintain.

The authors wish to acknowledge the financial support received from Enterprise Ireland, the Irish Research Council, Molloy Environmental Systems and Offaly County Council in the development of the Moneygall facility.