**1. Introduction**

The reuse of domestic wastewater (DW) in agricultural irrigation is seen as a fundamental alternative to alleviate water scarcity in the world [1], besides contributing to the reduction of environmental impacts, costs in treatment, and the discharge of DW to natural water bodies [2].

The availability of wastewater (WW) throughout the year is indicated as one of the most important aspects of water reuse in agriculture, which will reduce the dependence on precipitation, especially in regions with arid or semi-arid climates [3]. The recycling of nutrients available in the WW is an important economic alternative, as in many cases, it allows farmers to reduce or even eliminate the application of conventional fertilizers in their production fields [2]. In the literature, there are numerous experimental researchers in which WW has been used successfully for irrigation of agricultural vegetable crops [2, 4].

Despite the numerous benefits provided by effluent reuse in irrigation, it is important to emphasize that improper application can be harmful to plants, animals, producers, and consumers, as well as to the soil [5], because unlike drinking water, reuse water may contain high concentrations of bacteria, viruses, salts, and heavy metals, depending on its source and treatments [6].

The direct discharge of UTWW into soil and water bodies has negative impacts on the various components of the production process. Therefore, the combination of effluent reuse with appropriate treatment and recycling methods could be vital to provide a quality effluent for reuse, especially one of acceptable quality for agricultural irrigation. There are currently several alternatives or wastewater treatment technologies for reuse in irrigation, ranging from expensive and complex, to low cost and simple in structure, implementation, and maintenance. The selection of the effluent treatment technology to be adopted should respect issues related to the economic, social, and environmental conditions of the beneficiary community, as well as the quality recommended for the purpose it is intended to apply [7, 8].

The wastewater treatment technologies used by the sanitation companies are not feasible for low-income rural communities, due to the high cost of implementation, operation, and maintenance [9], as well as the large dispersion of the population in rural areas. Thus, it is important to develop decentralized, low-cost, and easyto-operate technologies for the treatment of DW effluents [10], and to provide optimum water supply for non-potable purposes, such as irrigation of agricultural crops.

There are several technologies that can be used for the treatment of DW for reuse in vegetable irrigation [11], such as: anaerobic filters (ANF), aerobic filters (AEF), septic tanks (ST), constructed wetlands (CWs), filter membranes, chlorination, sand filter, UV disinfection, among others. ST is the simplest and oldest low-cost system widely used as decentralized treatment technology, but, post-treatment is usually required because of the high soluble organic matter and pathogens content that remains in the effluent [12]. ST alone contributes to total suspend solids (TSS) (62%), chemical oxygen demand (COD) (31%) and fecal coliforms (FC) (31%) removal [13]. Similar results were presented by Del Castillo et al. [9], which found removal efficiency of 35% for COD, 73% for TSS, and 33% for biochemical oxygen demand (BOD). According to Bouted and Ratanatamskul [14], the aerobic treatment system has been recognized as a highly efficient system, but they have relatively high running costs in terms of energy. CWs are attracting interest as potential low-cost treatment solutions [15] and are practiced for primary and secondary treatment of DW [16]. However, when used alone to treat DW, CWs might not be able to meet quality guidelines for agricultural reuse [9].

In the ANF, the treatment process occurs in the absence of oxygen, and the highlight compared to other treatment systems according to da Silva et al. [17], is its high efficiency in the degradation of solid organic waste, converting organic matter into

#### *Anaerobic Filters: Alternative Solution for the Treatment of Domestic Wastewater for Reuse… DOI: http://dx.doi.org/10.5772/intechopen.106326*

biogas, which can be used for thermal, electrical or mechanical energy generation, biofertilizer or substrate that can be used to improve the physicochemical and biological properties of the soil. In general, the ANF removes total dissolved solids (TDS) and TSS through close contact with anaerobic bacteria attached to the filter media. Refers. [18] reported that ANF can present removal efficiencies higher than 70% for TSS and BOD, and 90% for fecal coliforms (FC).

Every treatment technology has different benefits, limitations, cost requirement and land area, payback period, and removal efficiency. Among all above mentioned technologies, ANF stands out because is a low-cost and sustainable treatment technology [18]. Tripathi et al. [19] also highlight that the operational simplicity is one of the main advantages of using ANF in wastewater treatment, specifically because they can be operated without the need for electricity, which makes them suitable for developing countries and rural communities or regions isolated from large urban centers. However, despite the diversity of their application and benefits provided, similarly to CWs, when ANF are used as a standalone technology, the effluent often does not meet the quality guidelines for agriculture reuse [9].

Abegunrin et al. [20] report the limitation of the application of ANF in the treatment of WW with higher concentrations of suspended solids, which is why it is commonly used for post-treatment. Tonon et al. [21] observed that the BOD and COD removal efficiency decrease with the increase of the hydraulic loading rate. According to de Oliveira Cruz et al. [18], nitrogen compounds and phosphorus concentration do not change during anaerobic treatment, thus, requiring an additional aerobic step or CWs to increase the quality of the final treated effluent. The improvement of the anaerobic (filter) system to become more efficient in treating DW is challenging, compared to the traditional aerobic treatment systems [22]. To deal with the limitations of the anaerobic (filter) DW treatment system, researchers are studying the performance of ANF in different environments [14], plant operational conditions [9], and alternative filled mediums [14] to identify the best way to improve their efficiency.

It is evident the need to study and develop low-cost technologies for WW treatment, which can contribute to the rational replacement of potable water, for treated DW in certain activities. It is also evident, the importance of discussing the need for the reuse of lower quality water in less sensitive activities concerning water quality, especially in irrigated agriculture. In this context, this research aims to evaluate the performance of ANF filled with gravel #1 (as inert material) in the treatment of DW for agricultural reuse.
