This study investigated down-flow hanging sponge (DHS) technology as a promising trickling filter (TF) using sponge media as a biomass carrier with an emphasis on protection of the biomass against macrofauna overgrazing. A pilot-scale DHS reactor fed with low-strength municipal sewage was operated under ambient temperature conditions for 1 year at a sewage treatment plant in Bangkok, Thailand. The results showed that snails (macrofauna) were present on the surface of the sponge media, but could not enter into it, because the sponge media with smaller pores physically protected the biomass from the snails. As a result, the sponge media maintained a dense biomass, with an average value of 22.3 gVSS/L sponge (58.1 gTSS/L sponge) on day 370. The snails could graze biomass on the surface of the sponge media. The DHS reactor process performance was also successful. The DHS reactor requires neither chemical treatments nor specific operations such as flooding for snail control. Overall, the results of this study indicate that the DHS reactor is able to protect biomass from snail overgrazing.
Trickling filter (TF) systems have several attractive advantages, such as simple operation, easy maintenance, low operation and maintenance costs, and low excess sludge production (Metcalf & Eddy 2003). TF systems have been used not only for secondary treatment, but also for tertiary treatment in terms of nitrification (Gujer & Boiler 1984; Gujer & Boller 1986; Parker et al. 1997; Gujer 2010).
One of the promising TF systems is the down-flow hanging sponge (DHS), which employs sponge media as the biomass carrier. To date, the DHS system has primarily been used for the post-treatment of anaerobic reactors treating municipal wastewater (Agrawal et al. 1997; Tandukar et al. 2007; Kassab et al. 2010; Onodera et al. 2014a). Unlike conventional packing media such as crushed stone and plastic (Lekang & Kleppe 2000; Metcalf & Eddy 2003; Wik 2003; Daigger & Boltz 2011), sponge media provides a three-dimensional space to retain biomass (Onodera et al. 2013). As a result, the retained biomass concentration can reach 26 gVSS/L (Onodera et al. 2013). The retained biomass consists of a variety of organisms including bacteria (Kubota et al. 2014), protozoans, and metazoans (Onodera et al. 2013).
However, macrofauna overgrazing of biomass in TF systems commonly occurs, resulting in disruption of the treatment process (Gujer & Boller 1986; Daigger & Boltz 2011). Various strategies such as physical (Hawkes & Shephard 1972; Parker et al. 1990), chemical (Parker et al. 1997), and combined physical and chemical applications (Parker et al. 1989) have been used to minimize the accumulation of macrofauna such as flies, worms, and snails (Boltz 2008). Indeed, macrofauna control, particularly snail control, has become an integral facet of modern TF design (Boltz 2008). However, Daigger & Boltz (2011) pointed out that many of these strategies have proven ineffective in some TF systems, and others may be detrimental to bioreactor performance. As for DHS technology, it has been reported that worms and fly larvae were present in a reactor that was used for the post-treatment of an up-flow anaerobic sludge blanket treating municipal sewage (Onodera et al. 2015). In contrast, there have been no reports of macrofauna overgrazing followed by process upset, despite many experiments and publications on DHS technologies. However, there is a general lack of any mechanism for protection against macrofauna overgrazing.
This study evaluates DHS technology with an emphasis on protection against macrofauna overgrazing. A pilot-scale DHS reactor was set up at an actual municipal treatment plant in Thailand and fed low-strength municipal wastewater under ambient temperature conditions for more than 1 year. Process performance was successful with regard to organic removal and nitrification (Onodera et al. 2014b). The DHS reactor was operated without specific operations for macrofauna control, such as chemical treatments and flooding.
MATERIALS AND METHODS
Reactor configuration and sponge media
The pilot-scale DHS reactor was installed at a sewage treatment plant in Thung Khru, Bangkok, Thailand (Figure 1). The DHS reactor was composed of two units in series, which each consisted of a rectangular-shaped segment 0.230 m long, 0.230 m wide, and 0.422 m high and were filled with sponge media to a height of 0.300 m. Each unit consisted of six segments (total height 1.800 m).
The sponge medium was made up of a plastic net-ring enclosure packed with polyurethane sponge (33 mm × 33 mm × 33 mm). The void ratio of the sponge was 97%, and the average pore size was 0.63 mm. Plastic net-rings were used to prevent the sponge from being compressed. The total volume of the sponge media was 50.5 L and the occupancy ratio was 53% in the sponge-filled portion. A settler was installed under the bottom of the DHS units, and activated sludge was used as a seed during the start-up period (Onodera et al. 2014b).
The DHS reactor was fed with actual municipal sewage after screening. The influent wastewater was fed into the top of the unit using a peristaltic pump and allowed to trickle down through the sponge media by gravity. The first-unit effluent was fed into the second unit. Oxygen was supplied by natural ventilation through small holes in the segments of the DHS reactor, and the sewage temperatures were in the range of 28–33 °C (average 30 °C) during operation. The hydraulic retention time (HRT) based on the sponge media volume was 1 h (flow rate: 2.4 m3/day). The influent biochemical oxygen demand (BOD) concentration was low. The operational data have been reported elsewhere (Onodera et al. 2014b).
Three sponge samples were taken from individual segments of each unit at random. The samples were squeezed and washed with distilled water until almost all of the biomass was removed, after which the biomass concentration was calculated based on the sponge volume. The three individual biomass suspensions were measured to determine the dispersion in the same segment on day 370. Biomass concentration was measured according to APHA (1998). The snails were observed using a magnifying glass.
Water quality analysis
To determine the water-quality profile, wastewater samples were collected along the DHS reactor height. Sampling was conducted at one time on day 300. The dissolved oxygen (DO) concentration was determined using a DO meter (DO-31P; TOA-DKK, Tokyo, Japan). The soluble samples were prepared using a 0.4-μm glass fiber filter (GB-140; Advantec, Tokyo, Japan). Chemical oxygen demand (CODCr), ammonium (NH4+), nitrate (NO3−), nitrite (NO2−), and total nitrogen (TN) were determined with a spectrophotometer (DR2800; Hach, Loveland, CO, USA) after pre-treatment according to the Hach. All other analytical procedures were performed according to APHA (1998).
Snails on the surface of the sponge media
Although a number of snails, identified as Physa acuta, were observed, they were particularly abundance in the upper segments of the first unit and tended to remain on the surface of the sponge media (Figure 1). The prevalence of snails was observed after the sponge biomass attained a mature stage i.e., at 6 months. These findings suggest that the DHS reactor prevents invasion of snails (size approximately 5 mm) into the sponge media (pore size: 0.63 mm on average). Worms and fly larvae were not dominant in the DHS reactor.
The biomass concentration of the DHS reactor increased gradually throughout the experimental period, attaining the average concentration of 22.3 gVSS/L sponge (58.1 gTSS/L sponge) on day 370 (Figures 2 and 3). The biomass concentration was comparable with that observed in previous studies, despite the low-strength wastewater (Tandukar et al. 2005; Onodera et al. 2013). The operational problems related to the biomass clogging and severe discharge were not observed during the experimental period.
The change in water quality along the reactor height at an HRT of 1 h is shown in Figure 4. Even though the DHS reactor was not provided with intentional aeration or a forced ventilation system, DO tended to increase from nearly zero at the inlet (wastewater) to approximately 4 mg/L after the second segment of the first unit (Figure 4(a)), and higher DO levels were maintained afterward. Organic compounds were primarily removed in the first unit and maintained at a stable level through the second unit (Figure 4(b)). In addition, nitrification occurred in the first unit (Figure 4(c)). Nitrate was detected in the wastewater stream. The total nitrogen was reduced from 9.4 to 3.9 mg/L in the DHS reactor, which is in accordance with the nitrogen concentrations observed at an HRT of 1 h (7.3 ± 1.8–4.1 ± 1.0 mgN/L) (Onodera et al. 2014b).
The pilot-scale experiment using actual municipal wastewater indicated that the sponge media was effective at protecting against snail overgrazing. The results give us a unique mechanism of DHS technology; that is, the small pores of the sponge media allow beneficial biomass grazing (excess sludge reduction) on the surface and protect the inside against snail invasion and overgrazing (Figure 5). The macrofauna may contribute to low excess sludge production and sufficient ventilation, resulting in efficient operation, especially with respect to nitrification (Hawkes & Shephard 1972). Indeed, the DHS reactor showed sufficient nitrification (Figure 4) and lower excess sludge production of 0.051 gTSS/gCOD removed for the last 2 months of the experimental period (Onodera et al. 2014b). When compared with other packing materials applied in TF systems (Lekang & Kleppe 2000; Daigger & Boltz 2011), sponge media is unique in that it provides three-dimensional space for biomass retention. Therefore, the DHS reactor allows for the increase in concentration of biomass inside sponge media (Figures 2 and 3), regardless of the presence of snails (Figure 1). This mechanism of the DHS reactor can be an attractive advantage over typical TF systems. It has been reported that nitrification was upset by Physa gyrina grazing on the biomass in a nitrifying tertiary TF using cross-flow plastic media (Palsdottir & Bishop 1997). In addition, the nitrification efficiency was drastically reduced by a massive invasion of fly larvae in a nitrifying tertiary TF (Gujer & Boiler 1984).
The use of sponge media requires no chemicals, specific equipment, or specific operating conditions. Conventional strategies in typical TF systems to control snails include periodic flooding, reducing distribution speed to create higher flushing rates, flooding and chemical application, chemical treatment, saline water dosing, and dosing with copper sulfate (Metcalf & Eddy 2003; Boltz 2008), most of which complicate design and operation and maintenance (O&M) and increase costs. The DHS technology is advantageous because of its simple O&M and no destruction of sensitive biomass during snail controls.
The main disadvantages of TF systems are short residence times, pumping costs, and biomass grazing by higher organisms (Boller et al. 1994; Wik 2003). Conversely, the DHS reactor showed a longer residence time by virtue of the high water-holding capacity of the sponge media (Onodera et al. 2014b). Moreover, high biomass concentrations may enable use of a more compact reactor. Indeed, the DHS reactor had a drastically higher biomass level of 7.5–15 gVSS/L reactor volume (15–30 gVSS/L sponge volume) than that of a previously described TF system (0.75 gVSS/L reactor volume; Albertson 1995). Overall, the DHS is a promising technology for municipal wastewater treatment.
This experiment verified that inside of the sponge media with a smaller pore size was protected against snail invasion and overgrazing. As a result, the sponge media maintained a high biomass concentration, resulting in good process performance. The snails could graze the biomass on the surface of the sponge media, resulting in excess sludge reduction. Overall, the use of sponge media as a biomass carrier is simple and effective at preventing snail overgrazing with no chemical treatment and specific operation and therefore has the potential for widespread application at wastewater treatment facilities.
We thank Pracha Sungket and Kiattichai Sungsom at King Mongkut's University of Technology Thonburi for the reactor operation and conducting routine experiments. This study was supported by a Grant-in-Aid from the Environmental Research and Technology Development Fund (K113002), Ministry of the Environment, Japan and the NIES advanced research program.