# Brown Coconut Husks as Media Within an Anaerobic Filter for Improving On-Site Wastewater Treatment
## 1 Introduction  
Communities and individual households which do not have access to centralized wastewater treatment plants are responsible for their own wastewater treatment via on-site systems such as septic tanks. It was estimated in 2017 that 1.5 billion people use septic tanks around the world [1]. Onsite wastewater treatment systems are common across the Pacific Islands [2, 3, 4]. In some countries like the Kingdom of Tonga, there is no centralized wastewater treatment system, so households utilize on-site treatment with 94% of households using septic tanks [5]. One of the challenges with septic tanks is that they rely on individual households for maintenance, and they are typically built below ground so it can be challenging to know how well they are performing. If sludge is not regularly removed, it builds up, which reduces the volume available for treatment and therefore reduces the hydraulic retention time (HRT). Septic tanks can also become undersized if the number of occupants within a household increases. There have been some concerns about how effective treatment is and whether septic tanks are a potential source of pollution for waterways [2, 4, 6, 7]. It is important that any process that improves the treatment performance of septic tanks is appropriate for the communities and households which require them. Low maintenance and affordability are two key considerations. One proposed solution to enhance the systeminvolves the construction of an anaerobic filter downstream of the septic tank [8; 9]. The benefit of an anaerobic filter is that the biomass is retained on the surface of media, which makes the system less susceptible to washout as a result of changes in flowrate from an individual household [8], as would be the case in Tonga. The combination of a septic tank and an anaerobic filter has been used at full scale in Brazil [10]. A key component of an anaerobic filter is the media which the biofilm attaches to [11]. From a treatment point of view, the media needs to provide a large surface area for biofilm development and be inert or organic material would leach, which would reduce treatment efficiency. Anaerobic filters can be at risk of blocking [11], so the physical size and shape of the media are important in facilitating the movement of wastewater through the filter bed. Ideally, the media would be readily available locally and affordable. A variety of media can be used, including inert materials such as lava stones [12] or plastic media [13; 14], but it is also possible to use crop residuals such as walnut shells [15] and coconut husks [16; 17; 18]. Coir fiber has also been used as an attachment medium [19]. Several studies conducted in Brazil have demonstrated that green coconut husks (_Cocos noticra_) can serve as a suitable surface for the biofilm to attach to [16; 17; 18]. This is a potential solution for Tonga where there is an abundance of coconut husks. However, in countries like Tonga, the majority of coconuts are harvested when they are brown rather than green. Brown husks are older, fully ripened, and have fallen to the ground (making them easier to harvest), having lower moisture content (35% [20]). Green husks, on the other hand, are younger and have higher moisture content (86% [20]), having remained attached to the trees. [PERSON] et al., [20] compared the performance of green and brown coconut husks in anaerobic filters and found that brown coconut husks removed more chemical oxygen demand (COD) from the wastewater. Further evaluation found that green coconut husks leached more COD than brown coconut husks, which explains this finding [20]. However, the data presented were only collected over two months of operation, so a more extensive study is required. The leaching of COD from coconut husks gives rise to concerns regarding the potential leaching of nutrients from this organic material [20], though this has not been extensively studied.