**2. Downflow hanging sponge system**

Aerobic biological treatment process can be traced back to the late nineteenth century. The biological process uses oxygen to break down organic contaminants and nutrients from wastewater. Oxygen is continuously mixed using aeration device (air blower or compressor) into the wastewater. Aerobic microorganisms then feed on the wastewater's organic matter converting it into carbon dioxide and biomass which is later removed. There are several types of aerobic treatment processes based on their designs such as fixed film system, continuous flow suspended growth aerobic system, retrofit aerobic system and composting toilets [4]. For this chapter, fixed film aerobic treatment systems called Downflow Hanging Sponge (DHS) and Trickling filter (TF) have been chosen and discussed. The rationale behind choosing these systems is the similarity in their working principle.

TFs is the second most widely used aerobic biological wastewater treatment system after activated sludge process (ASP) around the world [5]. TF is nonsubmerged fixed-bed, aerobic biological reactor which was applied for sewage treatment for the first time in England in 1893 [6]. Pre-settled wastewater is continuously trickled or sprayed from the top with the help of a rotating sprinkler. As the water moves through the pores of the filter, organics are aerobically degraded by the biofilm covering the filter material. The trickling filter consists of a cylindrical tank and is filled with different packing material such as stones, rocks, gravels or special pre-formed plastic filter media. Since couple of decades, various improvements have been made in TF and it has found its application as a combination unit with other treatment systems. There are 129 TFs in Latin America being operated as a post treatment unit of Upflow Anaerobic Sludge Blanket (UASB) [7]. UASB is an anaerobic treatment system originally developed for industrial wastewater treatment. With due course of time, it became popular in developing countries for domestic wastewater treatment due to its affordability, simple construction, easy operation and maintenance [6–8]. Recent studies on TF following anaerobic sewage treatment system revealed that 25% of UASB reactor employed TF as post treatment system [8–10]. The combination of anaerobic and aerobic treatment is advantageous, and this system is simpler than those involving ASP, and leads to much lower energy consumption [11]. The combination of UASB and TF exhibits the high treatability and also is economically advantageous over other treatment systems in developing countries. It has been adopted whenever compact systems were required.

Employing the working concept of TF, in 1995 a research team of Professor Hideki Harada came up with the first concept of promising sewage treatment technology referred to as a DHS system in Japan [12]. DHS system is comparatively a new aerobic, post-treatment process where a simple polyurethane sponge act as a medium for all biological removal processes. DHS system consists of the sponge modules arranged along its height unlike the TF which use gravel, plastics, rocks as supporting media. There are six variations of sponges developed and tested through the rigorous improvement in its shape, arrangements and packing method [13]. The first generation was cube type DHS (G1), second was curtain type DHS (G2), third

#### *Downflow Hanging Sponge System: A Self-Sustaining Option for Wastewater Treatment DOI: http://dx.doi.org/10.5772/intechopen.94287*

was similar to TF with sponge supported by polypyrene plastic net (G3), fourth was arrayed sponge type (G4), fifth was improved design of G2 and sixth was similar with G3 but with hardened sponges (G6). The other technical details of the configurations are discussed in other study [14].

The working principle of DHS system is similar to TF. The wastewater is supplied to the top of the DHS system with the help of distributor, which trickles down through the sponge module and finally exits the system through a clarifier at the bottom. The influent with its organic matter is trapped and flows down through the sponge modules in the reactor where the biomass within the sponge degrades the organic matter. External aeration is not necessary for the operation since there is natural diffusion of air as it flows through the DHS system.

For almost two decades, DHS system was researched as the post treatment of UASB for domestic wastewater and implementation of its full-scale have justified it [15–18]. Modifications to DHS systems are mainly conducted to eliminate the shortcoming of reactor and improve the nutrient and pathogen removal efficiencies [19, 20]. At early stages, DHS reactor was developed to treat domestic wastewater, however the potential of DHS reactor for treating different types of wastewater such as aquaculture [21], industrial wastewater [22–24] (textile, arsenic, rubber processing etc.) leachates [25] are other trends observed from literatures. The full scale and pilot scale DHS systems are being operating in Japan [10, 16], Thailand [26], India [18, 27, 28], Egypt [20], and Vietnam [22, 23] for various kinds of wastewater. Besides, DHS as a standalone bioreactor for rare metal recovery [29] phosphate recovery [30], gas scrubbing [31] and methane recovery [32] have also been investigated. Since, tremendous amount of researches are being carried out, more emphasis on its self-sustainability would help to validate its application for developing countries for domestic wastewater treatment.
