**3. Concept of wastewater treatment sustainability**

With the on-going stress on selection of WWTPs, sustainability assessment of wastewater has become standard in developed countries and aspiration for the developing countries [33]. The concept of sustainability of wastewater treatment plants is based on the observation that economy, environment and social well-being are interlinked. The term sustainability has various interpretations, however, the World Commission on Environment and Development (WCED, 1987) quoted "Sustainable Development is the development that meets the needs of the present generation without compromising the ability of future generations to meet their own needs" [34]. To assess the sustainability of a system, various dimensions based on the short- or long-term goals have been taken into consideration. From the classic definition of sustainability indicators, it should always incorporate the three main pillars of sustainability i.e. economic, environmental and societal for holistic assessment [35, 36]. In case of developing countries, the studies were more focused on the economic affordability, convenience of end user and stakeholder, health risks, technology sustainability, environmental impacts by products, natural resource optimization and sanitation [37–39]. This pertains to the fact that choosing sustainability indicators should be contextualized to the local requirements for the decision makers to ascertain WWTPs for specific areas.

Since there is no comprehensive definition of self - sustainable WWTP, it could be defined as "a state of treatment system which can sustain itself without or less use of energy or resources from external source without causing harm or less harm to the surrounding environment". This definition is restated with reference to the definition of appropriate technology for water sanitation for developing provided in these studies [40, 41]. The following could be some of the features that indicate the self-sustainability of WWTPs for developing countries;


Merely saying DHS is a 'sustainable'system is not possible until and unless sustainability indicators indicate progress towards or away from sustainability [42]. The main goal of this chapter is to present the state-of- - art of DHS system based on sustainability indicators. Additionally, the self - sustainability potential of DHS system was compared and discussed with the similar kind of technology i.e. TF for future application of this technology in developing countries. TF is a well-known technology since ages and comparing DHS system with TF would assist in its proof of concept, scalability and deployment for its validation in the field of the sustainability science. So far, there is only one study which has addressed the sustainability of the full-scale DHS system [43]. However, self-sustainability of DHS system has not been explored yet. From here on wards, DHS system is rephrased as UASB +DHS system as majority of researches on DHS system are presented as post treatment unit of UASB system. Similarly TF is also rephrased as UASB+TF. Apparently, the literatures on performance of UASB+TF systems are scarce so some discussions are presented with only TF data.

This chapter collects and analyzes the pre-requisites of self-sustainability indicators for UASB+DHS system. To address the self-sustainability of UASB+DHS system multiple indicators are considered from literature review for the holistic assessment which is guided predominantly by these studies [44, 45]. The indicators considered for this review are discussed henceforth and are summarized in **Table 1**.

#### **3.1 Treatment performance**

Right from the first prototype of UASB+DHS system, its treatment efficiency for organic, nitrogen and pathogen have shown impressive results for domestic wastewater treatment [10, 12–20]. There are plethora of studies reporting the treatment performance of UASB+DHS system. For comparison, the treatment efficiencies for the parameters such as Total suspended solids (TSS), biological oxygen demand (BOD), ammonia (NH4 + -N) and fecal coliform (FC) are collected and tabulated in **Table 2**. Full scale UASB+DHS system till date have shown significant TSS and BOD efficiencies of 94% and 96% respectively [18, 27]. While some of the selected UASB +TF system indicated a slightly reduced efficiency i.e. (TSS: 88–93% and BOD:89– 93%). For most of the developing countries, BOD standards are regarded as the


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


*Promising Techniques for Wastewater Treatment and Water Quality Assessment*

**Table 1.**

*Multiple indicators chosen for assessing the sustainability. Adapted from [44,*

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


#### **Table 2.**

*Treatment performance and land required for UASB+DHS system and UASB+TF system.* basic compliance discharge standard [12] which might have increased the popularity of UASB+DHS system. The data clearly shows that UASB+DHS system has benefits over UASB+TF system attributed by its unique sponges, improving the quality of effluent in terms of organic matter. Similarly, a noticeable ammonia removal efficiency ranging from 79–83% was showcased by UASB+DHS system whereas UASB+TF displayed decreased efficiency below 50%. Studies on molecular microbiology of UASB+DHS have highlighted slow growers such as nitrifiers, denitrifies and even active annamox bacteria in the inner aerobic niches of the sponges facilitating the nitrification and denitrification reaction for nitrogen removal [16, 51]. The other studies also reported that TFs have poor consistency in the removal of nitrogen and phosphorus compared to other conventional treatment systems [6]. Likewise, UASB+DHS system is also efficient for removing pathogenic bacteria from wastewater which was due to high DO condition which prevented growth of bacteria and allowed the higher micro-organisms (protozoa and metazoan) to predate on pathogens such as *E. coli* and total coliforms [52]. Moreover, the other factor for removal of pathogen in the UASB+DHS system reported was adsorption onto biomass [20]. While on the other hand, pathogen removal by UASB +TF system is promising in this case. However, the pathogen removal capacity in TFs have been observed inconsistent and varied from 1.0 log to 3 logs, depending on the operating conditions when compared to ASP [53].
