Anaerobic Filters: Alternative Solution for the Treatment of Domestic Wastewater for Reuse in Vegetable Irrigation

Abstract

This chapter is the result of a research conducted in the experimental area of the Department of Rural Engineering, School of Agronomic Sciences, São Paulo State University, Botucatu, São Paulo, Brazil, for 165 days, between the months of February and August 2018, with the objective to evaluate the performance of a wastewater treatment plant (WWTP) at a pilot-scale composed of six anaerobic filters (tank-in-series), filled with gravel #1 in the treatment of domestic wastewater (DW) for agricultural reuse. The parameters were monitored was: pH, electrical conductivity, total suspended solids, biochemical oxygen demand, chemical oxygen demand, total nitrogen, total phosphorus, and potassium. The results indicate that the WWTP performed satisfactorily and provided treated wastewater (TWW) with acceptable quality for agricultural reuse in irrigation of vegetable crops. It was observed that the mean concentrations of the pollutants decreased as wastewater advanced through the filters stage, presenting high removal efficiency at the 6th filter (TWW 6) than the 3rd filter (TWW 3), with statistical analysis corroborating that there are significant differences between the quality of TWW 3 and TWW 6 for most of the parameters evaluated, suggesting that the increase of the number of filters in this treatment system proposed improves the treated wastewater quality.

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 easy-to-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 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.

2. Materials and methods

2.1 Site description

The research was developed in the experimental area of the Department of Rural Engineering, School of Agronomic Sciences (FCA), São Paulo State University (UNESP), Botucatu, São Paulo, Brazil, at coordinates 22° 50′ 48″ S, 48° 26′ 06″ W and altitude of 817.74 m. The climate of the region is defined as type Cfa (Koppen): humid subtropical climate (mesothermal) with rainy summer and dry winter, the average temperature of the warmest month is above 22°C and the average annual precipitation around 1,501.4 mm.

2.2 Design and operation of the wastewater pilot-scale plant

The research comprised two phases; the first (Phase I) [23] the wastewater treatment plant (WWTP) had two beds, one filled with gravel #1 and other filled gravel #4. Each bed was composed of three vertical filters connected in series and operated for 105 days. Based on the results of Phase I, in the second phase (Phase II), the WWTP comprised a bed composed of six filters (Figure 1) made in 200 l plastic barrels (with dimensions of 0.90 m high and 0.50 m diameter), filled only with gravel #1, the water level was maintained at 10 cm from the barrel surface (to prevent overflow), providing an average porosity of 48% about the inert material, corresponding to a daily application rate of 95 liters and a hydraulic detention time (HDT) of 5.4 days.

Figure 1.

Frontal (a) and lateral (b) view of the WWTP set up at the FCA, UNESP, Botucatu.

The choice of the plastic barrel for making the filter was due to its low price and capacity to resist the weight of the inert material, weather conditions and the change in the structure of the WWTP in Phase II aimed to improve the quality of the TWW, because, the values of the BOD of the TWW obtained in the Phase I were above the range established for irrigation of food and non-food crops according to US Environmental Protection Agency (EPA) [8]; therefore, the increase in the number of filters aimed to increase the HDT and improve the performance of the WWTP, essentially for BOD and the other parameters associated with it.

The WWTP also had a 1,000-liter water tank (to receive and store the WW) and two 150-liter water tanks, one before (influent storage) and after (final TWW storage) the 6th filter, the second tank also served to store the TWW used in the irrigation system. The design of the system was proposed (Figure 2) in order to reduce the space requirement.

Figure 2.

General scheme of the WWTP (pilot-scale) set up at the FCA, UNESP, Botucatu.

DW (Table 1) used in this research came from SABESP´s Domestic WWTP in Botucatu city (secondary effluent). This WWTP presents a mixed treatment system, composed of equalization, Up-flow Anaerobic Sludge Blanket and activated sludge. The WWTP is responsible for the WW treatment of the urban area of the municipality of Botucatu, which is predominantly DW, with little contribution of industrial discharges.

	41 days	70 days	100 days	130 days	165 days	Average (SD)	Stand. for Agric. Reuse*
pH	7.87	8.17	7.5	6.9	7.03	7.49 (±0.62)	6.5–8.4a
EC	577	752	693	719.5	717.67	691.83 (±0.06)	≤700a^,d
TS	389.67	808	1270.83	1085.67	1291.67	969.17 (±377.62)
TSS	44.33	512.33	920.33	737.33	931	629.07 (±341.19)	<500a
TDS	345.33	295.67	350.5	348.33	360.67	340.1 (±25.50)	<450a; <500c
BOD	48	54	91	87	74	70.81 (±17.82	<10a; <30a
COD	600.8	445	1251	706.33	1000.8	800.8 (±299.4)
TN	76.03	65.6	96.7	77.8	84.03	80.03 (±11.13)	<10a
TP	10.56	13.76	7.92	13.18	12.68	11.62 (±3.01)
K	80	71.5	76.9	97.6	84	82.00 (±9.65)
TC	2,06 × 10^9	2,19 × 10^10	1,99 × 10^7	1,35 × 10^7	5,17 × 10^6	*
FC	7,42 × 10^8	Absent	1,37 × 10^6	2,36 × 10^6	3,36 × 10^5	*	≤1000b^,c

Table 1.

Characteristics of wastewater quality from SABESP´s domestic WWTP (influent in this research).

*

Standard water quality for agriculture reuse.

^a

Ref. [8].

^b

Ref. [7].

^c

Ref. [24].

^d

Ref. [25].

EC—electrical conductivity (μS cm⁻¹), TS—total solids (mg l⁻¹), TSS—total suspended solids (mg l⁻¹), TDS—total dissolved solids (mg l⁻¹), BOD—biochemical oxygen demand (mg l⁻¹), COD - chemical oxygen demand (mg l⁻¹), TN—total nitrogen (mg l⁻¹), TP - total phosphorus (mg l⁻¹), K - potassium (mg l⁻¹), TC—total coliform (MPN 100 ml⁻¹), FC—fecal coliforms (MPN 100 ml⁻¹). SD: standard deviation.

2.3 Water quality sampling

In Phase II, the research was conducted for 165 days. Samples were collected from the outlet of the influent (IF) storage, TWW released from the filter 3 (TWW 3) and filter 6 (TWW 6). The first sampling was performed 41 days after the beginning of the operation of the WWTP and the other samplings (4) were performed at 30-day intervals.

The WW quality analyses were performed in the Laboratory of Water Quality of the Department of Rural Engineering located at the School of Agronomic Sciences, UNESP, Botucatu.

All WW quality parameters such as total solids (TS), TSS, BOD, COD, total nitrogen (TN), total phosphorus (TP) and microbial population were determined according to the Standard Methods [26]. pH and EC values were measured in situ by digital pH and conductivity meter.

The microbiological indicator adopted for the fecal organisms was the concentration of total coliforms (TC) and Escherichia coli (E. coli), and its determination was performed using IDEXX Colilert Quanti-Tray System as per the manufacture´s procedures (https: //www.idexx.com/files/colilert-procedure-en. pdf, accessed on 13 June 2021). The most probable number (MPN) was used to find the concentration of TC and E. coli in the samples [27], expressed in MPN 100 ml⁻¹.

2.4 Statistical analysis

The results of the performance evaluation of the WWTP were submitted to analysis of variance to verify whether or not the change in the structure of the WWTP in Phase II had significant effects on the quality of the TWW, specifically in the parameters: pH, EC, TSS, BOD, COD, TN, TP and K. The statistical analysis considered two treatments, which comprised sample collection points, identified by: TWW 3 and TWW 6, with three repetitions per sampling, an entirely randomized design with repetitions in time. The variables that showed significant differences were subjected to Tukey’s test, at the 5% probability level, with the aid of the Software Sisvar version 5.6 [28].

3. Results and discussion

As shown in the Table 1, the quality of the influent used in this research does not meet the standard water quality recommended for reuse in agricultural irrigation.

The performance evaluation of the WWTP was essentially based on the comparison of the physical, chemical, and microbiological characteristics of TWW 3 and TWW 6, with IF. The main findings are presented and discussed below.

During the experiment, the maximum, minimum, and average temperatures were: 26, 16, 20°C, respectively; and the effluent operated in the range of 18–25°C. Thus, it can be stated that the filters operated between temperature ranges considered psychrophilic and mesophilic for microorganisms; the latter is considered by Chernicharo [29] as the one that anaerobic digesters have been commonly designed for because it presents the best results. Regarding the occurrence of rain, during the experimental period, a total of 230.3 mm was recorded, and it is believed that this did not influence the results because it did not coincide with the periods of sample collection. However, it can be recommended to place covers on the filters to prevent the entry of rainwater, and consequently compromise the microbial activity in the beds, which comprise the main agents of decomposition of organic matter.

Table 2 shows the average and standard deviation of the measurements corresponding to each sampling point and provides an overview of the overall performance system for the parameters pH, EC, TS, TSS and TDS. It can be observed that mean concentrations of EC, TS, TSS and TDS decrease as wastewater advances through the sampling points. The results of pH indicate that there was a tendency of increasing as WW advanced through the sampling points, however, they remained within the range considered adequate (pH between 6 and 8) for the anaerobic digestion [29]; they also remained within the adequate range (pH between 6.5 and 8.5) for reuse in crop irrigation [25].

According to the parameters proposed by Ayers and Westcot [25], TWW with EC less than 700 μS cm⁻¹ is not restricted from use in irrigation, even for salinity-sensitive crops. Thus, according to these parameters, the EC of the present research presents satisfactory quality for reuse in irrigation without restrictions.

The results of TS, TSS, and TDS, indicate that the WWTP performed satisfactorily in the reduction of solids. For example, an average reduction of 62.9% of ST and 99% of TSS was registered in TWW 3, 67% of TS, and 100% of TSS in TWW 6, concerning the concentration of these in the IF.

The levels of removal of TS and TSS obtained in this research were higher than those observed by Reinaldo et al. [30] evaluating a sewage treatment system consisting of a decant digester with a biological filter followed by constructed wetland and solar reactor, which obtained average efficiencies of removal of TS and TSS of 61 and 99% respectively; and [31] studying treatment system in ANF followed by two constructed wetlands, observed average efficiencies of removal of TS and TSS of 29 and 74% respectively. Fia et al. [31] associated the low efficiency of TS and TSS removal to low temperatures (average of 17.4 ± 2.2°C) recorded during the experimental period, which according to them, kept the fluid viscosity high, which resulted in lower sedimentation velocity of the biomass produced.

Low concentration of TSS observed in this research can be beneficial in rural areas for reuse in agriculture, since the chances of clogging irrigation equipment, such as emitter, would be lower. According to Lamm et al. [32], when the TSS concentration is less than 50 mg l⁻¹ there is a lower risk of dripper clogging.

3.1 Chemical and biochemical oxygen demand

The efficiency of the WWTP in the reduction of organic matter can be considered satisfactory; because, an average reduction of BOD and COD was observed in the order of 33.3 and 96.2% in TWW 3, 62.9, and 96.7% in TWW 6 respectively; and registering average values of BOD and COD in TWW 6 of 26.24 mg l⁻¹ and 26.78 mg l⁻¹ respectively (Figure 3).

Figure 3.

BOD (a) and COD (b) concentration and remove efficiency comparison during the operation of WWTP.

According to EPA [8], BOD values of irrigation water for crops consumed cooked and raw should not exceed 30 and 10 mg l⁻¹ respectively. Since kale is a vegetable that is commonly consumed cooked, it is observed that only TWW 6 meets the required quality standard, with a mean value of 26.24 mg l⁻¹. These results show that the WWTP was not only a filter-in-series but a biological treatment unit as well. One way of demonstrating that the treatment provided by WWTP proposed in this research was not restricted to physical removal by filtration, is the evaluation of the BOD concentrations. de Oliveira Cruz et al. [18] observed COD removal of 41% (lower than our results), and assumed that their anaerobic filter was not restricted to mechanical actions such as fixation, interception, and adsorption, given the fact that the degradation and consumption of soluble material also occurred, by the action of the biofilm present in the interstices of the filled medium, this same behavior was observed by Tonon et al. [21].

Tonetti et al. [33] obtained BOD reduction values higher than 98% studying an alternative WW treatment system consisting of an ANF (with bamboo support material) followed by a sand filter. In this system, they observed that the ANF provided a BOD reduction of 47%. The same authors mention that although ANF usually present good removal efficiency, between 10 and 30% of organic matter is not degraded, which limits that their WW meets the quality standard required by the Brazilian legislation, and this need posttreatment.

Hashem and Qi [34] states that the COD/BOD ratio is an indication of the amount of non-biodegradable organic matter. The higher this ratio is, it is an indication of low organic matter degradation that may be related to the failure of the biological treatment system adopted or the quality of the sewage. The ratio (COD/BOD) with a value of 1.02 recorded in this research is lower than that obtained by Colares and Sandri [35] who obtained 2.08 and observed BOD removal efficiencies in the order of 79.01% evaluating the performance of ST followed by constructed wetlands in the treatment of domestic sewage. Therefore, the low reduction of BOD observed in this research compared to the results found in the literature can be associated with the quality of the sewage and not necessarily the capacity of organic matter degradation of the WWTP of this research.

Reduction efficiencies of BOD and COD lower than our results were observed by Fia et al. [31], obtained 55 and 35% removal of COD and BOD respectively, operating a treatment system in downflow ANF followed by constructed wetlands systems. These authors, in common, indicated the quality of the raw sewage (low organic load) as the main reason for obtaining unsatisfactory results.

Although the BOD reduction was relatively low compared to what is found in the literature, it is important to mention that on average, the results obtained to meet the [8] requirements for irrigation of cooked consumed crops and the probable range of pollutant removal (represented by BOD) established in NBR 13.969 for ANF [36].

3.2 Total nitrogen, total phosphorus, and potassium

The performance of the WWTP regarding the reduction of TN, TP, and K indicated a removal efficiency of 70,2, 46.5, and 80%, resulting respectively in mean concentrations in the TWW6 of 23.83 mg l⁻¹ of total nitrogen (Figure 4a), 6.22 mg l⁻¹ of total phosphorus (Figure 4b) and 16.40 mg l⁻¹ of potassium (Figure 4c).

Figure 4.

TN (a), TP (b) and K (c) concentration and remove efficiency comparison during the operation of WWTP.

Bueno et al. [37] refers that fixed bed ANF or operated under static conditions and without recirculation are commonly unable to remove phosphate and nitrogen compounds efficiently. Bastos et al. [38] refer that the fact that anaerobic treatment occurs in the absence of oxygen is also a limiting factor for nitrogen reduction because nitrification requires the presence of dissolved oxygen, which is only possible in an aerobic environment. Therefore, comparing the results obtained in this research with those found in the literature, it can be seen that the system generated a satisfactory reduction of these elements.

Ucker et al. [39] refer that the removal of nutrients in effluents is more effective in treatment systems that involve the cultivation of plants. These authors, in their research evaluating a CW’s system using vetiver grass, they observed efficiencies of phosphorus and ammonia nitrogen reduction around 80.35 and 83.3% in modules with plants and, 44.45 and 42.55% in modules without plants respectively; the removal of phosphorus in modules without plants, was close to that obtained in the present research (46.5%).

The mean value of TP concentration observed during the research is within the range indicated by Bastos et al. [38], which varies between 4 and 12 mg l⁻¹; and TN is slightly above the range indicated by the same authors (35–60 mg l⁻¹), for sewage considered domestic. Comparing the characteristics of TWW 6 to the standards established by Brazilian and international guidelines, it was found that the average concentration of TN is in the range considered of moderate restriction for use in irrigation. Phosphorus exceeded the limits established by CONAMA Resolution 357/05 [24]; and potassium was not the target of comparisons because it is not included in any of the guidelines adopted for this purpose, although it is indicated by them as an element to be considered in analyses to determine the quality of water for irrigation.

3.3 Overall evaluation of the treatment system

The overall evaluation of the performance of the WWTP based on the results of the statistical analysis (Table 3) indicates there are significant differences in the quality of TWW 3 and TWW 6 for most of the parameters evaluated, demonstrating that changing the structure of the WWTP, specifically increasing the number of bed filters and consequently increasing the HDT in Phase II had a positive effect on the reduction of pollutants present in the IF. The positive effect of increasing the HDT on the quality of the TWW was also observed by Bouted and Ratanatamskul [14], in which operating isolated ANF in the treatment of influent under different temperatures and HDT (9, 18, and 27 h) obtained greater reductions of TSS, COD, PT and NT in the treatment system with the highest HDT (Table 4).

	41 days	70 days	100 days	130 days	165 days	Average (SD)
	pH
IF	7.87	8.17	7.5	6.9	7.03	7.49 (±0.62)
TWW 3	8.1	8.07	7.7	7.7	7.1	7.73 (±0.38)
TWW 6	8.13	8.3	7.8	7.7	7.5	7.89 (±0.30)
	Electrical conductivity (μS cm^−1)
IF	577	752	693	719.5	717.67	691.83 (±0.06)
TWW 3	592	721.67	677	711	684.67	677.27 (±0.05)
TWW 6	556.67	674.33	630	595.5	611	613.5 (±0.04)
	Total solids (mg l^−1)
IF	389.67	808	1270.83	1085.67	1291.67	969.17 (±377.62)
TWW 3	357.5	377.33	368.67	350.33	345.67	359.9 (±13.04)
TWW 6	346.66	348.66	285	320.33	304.33	321 (±27.38)
	Total suspended solids (mg l^−1)
IF	44.33	512.33	920.33	737.33	931	629.07 (±341.19)
TWW 3	7	2.33	9.67	9	2.67	6.13 (±3.23)
TWW 6	4.4	0.33	3.67	1.67	0.33	2.1 (±2.21)
	Total dissolved solids (mg l^−1)
IF	345.33	295.67	350.5	348.33	360.67	340.1 (±25.50)
TWW 3	350.5	375	359	341.33	343	353.77 (13.78)
TWW 6	342.17	348.33	281.33	318.67	304	318.9 (27.57)

Table 2.

Average values of water quality parameters measured at each sampling point.

IF: is influent; TWW: 3 is treated wastewater collected in filter 3; TWW: 6 is treated wastewater collected in filter 6; SD: standard deviation.

Parameters (Units)	P-value (F test)	MSD	Water quality per filter stage		CV (%)
			TWW 3	TWW 6
pH (μS cm^−1)	0.2308 ns	0.16	7.73a	7.89a	4.39
EC (mg l^−1)	0.0006**	9.09	677.3a	613.5b	6.96
TSS (mg l^−1)	0.0004**	0.91	6.13a	2.10b	67.2
BOD (mg l^−1)	0.0000**	1.09	47.16a	26.24b	31.85
COD (mg l^−1)	0.1091 ns	4.64	30.82a	26.78a	23.25
TN (mg l^−1)	0.0000**	3.27	36.23a	23.83b	14.75
TP (mg l^−1)	0.1076 ns	1.97	8.09a	6.22a	43.07
K (mg l^−1)	0.0000**	1.28	23.43a	16.40b	16.02

Table 3.

Average values and statistical analysis results of physical and chemical water quality.

**

Significant at 1% (P < 0.01).

TWW 3: is treated wastewater collected in filter 3; TWW: 6 is treated wastewater collected in filter 6. MSD: minimum significant differences; CV: coefficient of variation. Means that do not share the same lower-case letter in the row are significantly different, by the Tukey’s test; ns: not significant at 5% (P < 0.05).

	41 days	70 days	100 days	130 days	165 days
Total coliforms (MPN 100 ml^−1)
IF	2.06 × 10^9	2.19 × 10^10	1.99 × 10^7	1.35 × 10^7	5.17 × 10^6
TWW 3	2.16 × 10^5	3.26 × 10^6	6.65 × 10^5	7.82 × 10^5	1.08 × 10^6
TWW 6	1.45 × 10^4	2.76 × 10^5	1.24 × 10^3	7.70 × 10^3	4.35 × 10^3
E. coli (MPN 100 ml^−1)
IF	7.42 × 10^8	Absent	1.37 × 10^6	2.36 × 10^6	3.36 × 10^5
TWW 3	Absent	1.99 × 10^5	4.10 × 10^3	7.40 × 10^4	Absent
TWW 6	5200	Absent	21	328	97

Table 4.

Total coliforms and E. coli concentration comparison during the operation of WWTP.

IF is influent; TWW 3: treated wastewater collected in filter 3; TWW 6: treated wastewater collected in filter 6.

Another fact that should be highlighted is concerning BOD because, in average terms, only TWW 6 with an average BOD of 26.24 mg l⁻¹ presented quality suitable for irrigation of food and non-food crops (BOD less than 30 mg l⁻¹) according to EPA [8].

The effect of the changes introduced in the WWTP layout in Phase II is best evidenced by the microbiological characteristics of the TWW; for, although removal efficiencies of TC and E. coli around 100% have been recorded in TWW 3 and TWW 6 concerning the IF, only in TWW 6 did it reach acceptable quality standards for reuse in the irrigation of vegetables consumed raw or fruits that develop in contact with the soil, and in their consumption, the outer skin is not removed.

Based on the results presented in Table 4, it is observed that the quality standards established for reuse of TWW in irrigation according to World Health Organization (WHO) [40] and CONAMA Resolution 357/05 [24] are only met in the effluent collected in TWW 6, which from the third collection (100 days after the start of operation of the WWTP) showed a population of fecal coliforms below 1000 MPN 100 ml⁻¹; while for TWW 3, the E. coli population remained above 1000 MPN 100 ml⁻¹ in all samplings performed.

Figure 5 shows a visual indicator of the level of treatment obtained in the ANF WWTP implemented in the experimental area of the Department of Rural Engineering, School of Agronomic Sciences, Botucatu Campus. The figure shows that the samples of TWW 3 already indicated satisfactory levels of solids and organic load reduction.

Figure 5.

Photos of water aesthetics (clarity) from different points of samples collected. (a) Sample of influent; (b) Sample of TWW 3; (c) Sample of TWW 6.

Good water aesthetics can be a result of low turbidity and TSS concentration, which is important when the aim is reuse. According to de Oliveira Cruz et al. [18], these characteristics favor a more aesthetically pleasing TWW with fewer suspended solids to shelter pathogenic microorganisms, thus, decreasing the amount of chemical to clean it.

4. Conclusions

The WWTP with filter-in-series showed satisfactory performance for most of the parameters evaluated and provided final TWW with acceptable quality for irrigation of cooked consumed vegetables.

The results of the statistical analysis showed that there are significant differences in the quality of TWW 3 and TWW 6, and a lower concentration of salts, solid particles, organic load, and nutrients were observed in TWW 6.

The mean concentrations of the pollutants decrease as wastewater advances through the filters stage, indicating that the increase in the hydraulic retention time improves the performance of the WWTP.

TWW 6 presented good water aesthetics as a result of the low turbidity and TSS concentration, an aspect that is important when the aim is reuse.

The change in the structure of the WWTP, increasing the number of filters from three to six filled with gravel #1, improved the capacity of the proposed system to reduce the concentration of pollutants present in the IF.

It is recommended that the TWW 6 be used for irrigation 100 days after the start of operation of the WWTP proposed in this research because it was only after the third sampling that the final TWW meets the water quality standards for irrigation established by Brazilian and international guidelines.

Acknowledgments

The authors acknowledge the scholarship provided by the Ministry of Science, Technology and Higher Technical Professional Education (Ministério da Ciência e Tecnologia Ensino Superior Técnico Profissional-MCTESTP, Mozambique) and funding provided by the Coordination for the Improvement of Higher Education Personnel (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior—CAPES, Brazil).

Conflicts of interest

There are no conflicts to declare.

Author contributions

V.S.J.P. designed and conducted the experiments and prepared the original manuscript draft. R.M.S.-R. and J.G.T.Q. reviewed the manuscript. V.S.J.P. and T.L.S. collected and analyzed treated wastewater samples. S.A.J. and V.S.J.P. were involved in data preparation and statistical analysis.

Funding

This research was funded by Ministry of Science, Technology and Higher Technical Professional Education (Ministério da Ciência e Tecnologia Ensino Superior Técnico Profissional-MCTESTP, Mozambique) and Coordination for the Improvement of Higher Education Personnel (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior- CAPES, Brazil).