Assessment of Faecal Sludge Quality, Heavy Metal Pollution, and Ecological Risk: Implications for Sustainable Agriculture

doi:10.21203/rs.3.rs-4660652/v1

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Assessment of Faecal Sludge Quality, Heavy Metal Pollution, and Ecological Risk: Implications for Sustainable Agriculture

https://doi.org/10.21203/rs.3.rs-4660652/v1

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Pit latrines represent the predominant form of onsite sanitation in Botswana, posing unique challenges in faecal sludge (FS) management. The key concerns revolve around FS extraction, treatment, and safe disposal. Currently, co-treatment with wastewater is the primary approach, but it strains wastewater treatment plants (WWTPs) and compromises effluent quality. This study comprehensively assesses FS quality from pit latrines and evaluates potential health risks when used in agriculture for soil improvement. Systematic sampling at various depth intervals, approximately 30 cm thick, was conducted, followed by extensive analysis, including heavy metals (copper, iron, lead, cadmium, zinc, manganese, and arsenic).

The findings unequivocally demonstrate that FS from VIP latrines poses no significant health risks due to heavy metal content. Specifically, Geo-accumulation Index (Igeo) values for nickel (Ni), chromium (Cr), and arsenic (As) were consistently below zero, indicating negligible risk of environmental contamination. However, copper (Cu) exhibited Igeo values above zero, with a moderate pollution risk but within manageable limits. The high nutrient content, particularly of nitrogen and phosphorus, highlights its agricultural potential, though prudent management is needed to mitigate eutrophication. The study advocates for separate FS treatment, resolving co-treatment operational challenges and enhancing sustainability. Implementing these recommendations promises to address FS management issues, bolster food security, and enhance Botswana's ecological well-being.

pit latrines

faecal sludge

heavy metals

agriculture

wastewater treatment

ecological well-being

In pursuit of enhancing agricultural productivity around major settlements in Botswana, the government has allocated funding to support small-scale horticultural projects near the Gaborone Wastewater Treatment Plant (GWWTP) in Gaborone [Odirile et al., 2018]. This initiative addresses the challenge posed by the generally poor physical conditions of soils in semi-arid regions like Botswana, characterized by limited water retention capacity and low plant nutrient content [Odirile et al., 2018].. The conventional use of commercial fertilizers to bolster agricultural production has proven effective but is accompanied by increased production costs [Hammer & Hammer, 2014]. As a sustainable and efficient alternative, the utilization of sludge as fertilizer offers promise in restoring nutrients to agricultural soils [Malkki, 1999].

Wastewater treatment processes generate sludge as a by-product, which consists of organic and inorganic materials separated from the incoming wastewater through a combination of mechanical, biological, and chemical treatments [Hammer & Hammer, 2014]. However, this sludge can contain hazardous substances, such as heavy metals, micro-pollutants, and pathogens, posing potential risks to human health and the environment [Gimeno-García et al., 1996; Hashem, 2000]. Research has shown that using sludge as a soil amendment can increase the levels of certain metals, including cadmium (Cd), nickel (Ni), copper (Cu), and zinc (Zn), in crops such as wheat, potatoes, lettuce, red beets, cabbage, and ryegrass [Jiang et al., 2014]. Notably, lead (Pb) tends to remain relatively unavailable to crops from the soil [Wuana & Okieimen, 2011]. Additionally, the availability of metals to crops is reportedly lower in soil treated with dried sludge compared to liquid sludge [Zhang et al., 2010].

In light of these considerations, human faecal sludge from Ventilated Improved Pit latrines (VIP latrines) emerges as a viable alternative fertilizer due to its rich nutrient content and its ability to enhance soil quality [Nikiema et al., 2013]. While human faeces have been recognized as a valuable nutrient source in several countries worldwide, including China, Japan, Korea, and various African and South American nations, its acceptance in Botswana remains limited, primarily utilized by select urban residents for landscaping and gardening purposes [Jönsson et al., 2004]. Nevertheless, it is essential to acknowledge that faecal material represents a critical threat to human and animal health, as well as ecosystem integrity [Graham & Polizzoto, 2013].

Pit latrines are a global sanitation solution, serving approximately 1.77 billion people as their primary sanitation method [Diener et al., 2014]. They offer cost-effective, water-efficient, and low-maintenance sanitation, particularly valuable in water-scarce regions such as Botswana [Dzwairo et al., 2006]. The utilization of pit latrines has significantly improved sanitation conditions in developing countries, particularly in preventing parasitic and bacterial infections among children and infants [Carr & Strauss, 2001].

However, the management of faecal sludge, including pit emptying, transport, treatment, and disposal, poses complex challenges [Graham & Polizzoto, 2013]. According to previous research, the annual quantity of sludge generated from pit latrines averages around 520 kg per person, primarily comprising urine and faeces [Jacks et al., 1999]. Urine is rich in nitrogen, while faeces contain substantial phosphorous and potassium levels [Jönsson et al., 2004]. Recycling this sludge into the soil can replenish these essential nutrients, sustaining land fertility and agricultural productivity [Jacks et al., 1999]. Moreover, faecal sludge boasts a low content of heavy metals, a marked contrast to inorganic fertilizers, which frequently contain elevated heavy metal levels [Nziguheba & Smolders, 2008]. Notably, phosphate fertilizers, widely used in agriculture, often contain arsenic (As), cadmium (Cd), and lead (Pb) as inherent components of phosphate rock ore or other ingredients, resulting from the phosphate fertilizer industry's processes [Macedo et al., 2009]. The application of faecal sludge to soil, therefore, poses minimal threats related to heavy metal pollution, assuming it remains uncontaminated by industrial wastewater [Nziguheba & Smolders, 2008].

The current sanitation management paradigm involves the emptying of pit latrines, sludge transportation, and subsequent disposal, treatment, or reuse. Notably, heavy metal pollution within faecal sludge poses potential risks to human health, as these pollutants can be transferred to humans through crop consumption. Furthermore, the long-term application of untreated sludge on farmlands can diminish soil buffering capacity, thereby jeopardizing ecological environments [Jiang et al., 2014].

The primary aim of this study is to evaluate the biological and chemical properties of faecal sludge and to assess the potential risks and benefits associated with the reuse of VIP sludge in agriculture. The investigation encompasses the determination of metal content using Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) and the analysis of nutrients, including nitrate (NO₃^-), nitrites (NO^2-), and phosphates (PO₄^3-), using Ion Chromatography (IC) [Jiang et al., 2014]. The collected data will offer critical insights into the feasibility and safety of utilizing VIP sludge as a valuable resource within the context of Botswana's agricultural and environmental sustainability.

1.1 Research Context and Novelty

The study focuses on Botswana, a region with limited research on faecal sludge quality, heavy metal pollution, and ecological risk. While faecal sludge has been studied in various contexts, its quality, heavy metal content, and ecological implications have not been extensively explored, especially in relation to agricultural use. The detailed assessment of heavy metal concentrations and their potential ecological risks in faecal sludge is a unique aspect of this study, shedding light on a less-explored area of environmental concern. This research explores the potential of using faecal sludge as a sustainable agricultural resource, aligning with the increasing interest in eco-friendly farming practices and resource optimization [Zewde et al. 2021].

This study was conducted within the borders of Botswana, a landlocked country situated in southern Africa. Botswana is geographically surrounded by Namibia to the west, Zambia to the north, Zimbabwe to the northeast, and South Africa to the south, as depicted in Fig. 1.

The research specifically focused on two distinct regions within the capital city of Botswana, Gaborone, namely Mogoditshane and Broadhurst. These two areas were selected to represent varying socio-economic settings within the city. Mogoditshane, characterized by an unplanned rural village setup, stands as an exemplar of community-driven pit latrine construction with limited supervision. In Mogoditshane, the pits typically range in depth from 1.3 to 2.8 meters, with an average depth of approximately 1.83 meters. On the other hand, Broadhurst, located in a peri-urban context, presents a different scenario. The depth of sludge within the pits of Broadhurst depends on the duration of toilet use.

Gaborone serves as both the capital and the most populous city in Botswana, boasting a population of approximately 231,626 residents according to the 2011 census, constituting about 10% of the nation's total population. The geographic coordinates of Gaborone City are situated at 24°40' South latitude and 25°55' East longitude. Notably, faecal sludge originating from Mogoditshane and Broadhurst undergoes core treatment at the Gaborone Wastewater Treatment Plant, situated approximately 10 kilometers northeast of Gaborone City, Botswana.

3.1. Sludge sample Collection and Preparation

Overall 50 pits were studied and sampled in two localities of Gaborone (25 pits in Mogoditshane and 25 pits in Broadhurst) which represented different socio-economic setups. Samples were obtained from the pit using a Multistage Sludge Sampler through either the pedestal hole or the inspection chamber as shown in Fig. 2.

Sludge samples (about 3 kg) were collected in polyethylene containers and placed in a cooler box immediately after measurement of temperature and pH. They were then transported to the cold room at the University of Botswana in the Chemistry Department, where they were stored at a temperature of 4\(\:℃\). For chemical analysis samples were taken from the cold room and air-dried for five days to let water evaporate from the sample and also to avoid microbial action. After drying the samples, they were crushed with pestle and mortar and sieved to a size of 150 µm for particle size homogeneity and easy sample dissolution. The sieved samples were thereafter stored in sealed bottles at room temperature until the day of analysis.

3.2. Analytical Methods

3.2.1. Metal Analysis by ICP-OEP

Faecal sludge samples were securely transported to the Water and Environmental Engineering Laboratory of the University of Botswana and later to the Chemistry where they were analyzed for heavy metals (such as copper (Cu), iron (Fe), lead (Pb), cadmium (Cd), zinc (Zn), manganese (Mn) and arsenic (Ar)) and Microbial parameters (such as, Ascaris) respectively. All analyses of faecal sludge samples were conducted following the methods outlined by APHA/AWWA/WEF [2005] and Reddy [2013]. Faecal sludge sampling protocol were followed carefully to prevent contamination during sampling and transportation which may affect the results. The protocols used in the study were approved by the Departmental Ethics Committee. Briefly, 0.5 g of each sample was placed in a pre-washed conical flask. 100 mL of double distilled deionised water (DDW, 18 MΩ cm^− 1) and 12 mL of concentrated HNO₃ (analytical grade) were added to digest the sample. The samples were heated until the solution volume was reduced to approximately 0.5 mL. Samples were then cooled to about room temperature. After digestion, the residue was allowed to air-cool, then DDW was added. The obtained solution was filtered through a Whatman No. 42 filter paper and quantitatively transferred to a 25 mL volumetric flask and diluted with DDW to a final volume of 25 mL. Each sample was analyzed in triplicate for Cd, Cr, Cu, Ni, Pb, Mn, Sn, Fe, Zn, As, Na, K, Mg and Ca using ICP-OES. The results shown here represent layers one and two.

All reagents used were of analytical grade and deionized water (18.2 MΩ cm) from a Millipore Milli Q system. All the extraction procedures were performed using laboratory glassware and polyethylene bottles pre-cleaned with HCl and rinsed with double distilled water.

3.3 Data Analysis

The study analyzed the results by calculating the means, standard deviations, and p-values for both faecal sludge samples collected from peri-urban and rural areas. Microsoft Office Excel 2007 was used for this data analysis.

To evaluate the extent of heavy metal pollution in faecal sludge, the Geo-accumulation Index (Igeo) proposed by Muller, [1979] was employed, as depicted in Eq. 1:

Where C_n is the measured heavy metal concentration in mg/kg and B_n = geochemical background value, mg/kg. To determine the pollution level of the sample, the geo-accumulation index is classified in Table 1. A factor of 1.5 is introduced to minimize the potential impact of variations in the background values, which may result from lithologic differences in the sediments (Muller, 1979; Nowrouzi and Pourkhabbaz, 2014).

Table 1

Contamination categories based on geo-accumulation index [Nowrouzi and Pourkhabbaz (2014)]
index	Category	Description
Geo accumulation index (I_geo)	≤ 0	Practically Unpolluted
	> 0 to ≤ 1	Slightly polluted
	> 1 to ≤ 2	Moderately polluted
	> 2 to ≤ 3	Moderately to strongly polluted
	> 3 to ≤ 4	Strongly Polluted
	> 4 to ≤ 5	Strongly to very strong
	> 6	Very Strong

4.1. Nutrient Content in FS

The effects of organic matter, nitrogen, phosphorus and toxic elements in sewage sludge applied to agricultural land have been reviewed extensively in the literature. However, that effect is still limited in terms of Pit latrine/Faecal sludge which is rich in organic matter and may improve the structure and water holding capacity of poor soils as well as containing agronomically significant amounts of nitrogen and phosphorus in sludge to render it of fertilizer value [Malkki, (1999)].

Results in Fig. 3 and Table 2 revealed that the amounts of NO₃^-, NO₂^- and PO₄^3- in pit latrine sludges studied were very high. For example, in Mogoditshane, nitrates concentrations were as high as 4.47 x10⁴ mg/kg. These are important nutrients in crop production. The results show that in all pits investigated in Mogoditshane, average nitrite concentrations were higher than nitrate concentrations, but this was reversed in samples collected from Broadhurst pits. The average phosphate concentration in the two sampling areas were comparable, as high as 39000 mg/kg. Following the principles of sustainable development, nutrients in faecal sludge should be used in plant production, instead of ending up in wastewater treatment plants. However, the most undesirable consequence of such high nutrient concentrations is the risk of pollution which is posed by pit latrines. Several researchers have found that pit-latrines are a source of nitrate contamination and therefore a hazard to groundwater due to their huge capacity to play a part in chemical and/or microbial pollution [Graham & Polizzoto, 2013; Jacks et al., 1999; Jönsson et al., 2004; Nikiema et al., 2013; Appiah-Effah et al., 2015; Gimeno-García et al., 1996; Mafa, 2003)]. Earlier studies carried out by Mafa [2003] in the city of Francistown in Botswana showed that pit-latrines were found to have the highest impact on groundwater quality, resulting in such groundwater being unsuitable for consumption. Nitrogen (in the form of nitrate) is the most dominant of all these and is therefore used as a key indicator of overall groundwater quality [Graham & Polizzoto, 2013; Jacks et al., 1999; Jönsson et al., (2004)]. Moreover, it has been shown that in Botswana about 50% of nitrogen from pit latrines leaches to groundwater [Jacks et al., (1999)].

Table 2

Minimum, maximum and average values of NO₃^-, NO₂^- and PO₄^3- in pit-latrine sludge sampled from Mogoditshane and Broadhurst, analysed by Ion Chromatography.
Analyte (x10³ mg/kgdwt)	Location	Minimum (mg/kgdwt)	Maximum (mg/kgdwt)	Mean + - SD (mg/kgdwt)
[NO₃^-]	Mogoditshane	9.93 ± 0.14	125.22 ± 1.14	42.47 ± 3.21
[NO₃^-]	Broadhurst	6.06 ± 0.19	92.87 ± 3.03	46.33 ± 1.31
[NO₂^-]	Mogoditshane	5.96 ± 0.21	159.01 ± 3.75	59.61 ± 2.62
[NO₂^-]	Broadhurst	9.77 ± 3.19	149.03 ± 7.16	33.73 ± 2.03
[PO₄^3-]	Mogoditshane	4.28 ± 0.79	134.22 ± 2.28	38.02 ± 1.13
[PO₄^3-]	Broadhurst	5.56 ± 0.28	121.09 ± 6.59	39.52 ± 6.22

The results from all the pits were pooled and averaged per sampling location. These results indicated high average concentrations of all the nutrients above recommended maximum level that could contaminate ground water. In the area where sampling was carried out, there are boreholes in close proximity to several pit latrines. Previous research have shown that such close distances between groundwater sources and pit latrines always lead to chemical and/or microbiological contamination of groundwater [Graham & Polizzoto, 2013; Nikiema et al., (2013)], leading to nutrient values above the Maximum Allowed Limit (MAL) of 50mg/L set by [WHO, (2011)]. In addition to the fact that the soils in Botswana are permeable and thus allow for excessive bacterial and chemical pollution [Vogel, 2002], almost all the pits in the area on which we sampled from were not lined internally, thereby presenting little or no impediment to the mobility of bacteria and other fecal contaminants. The observed high variability (between minimum and maximum amount) observed could be attributed to poor mixing within layers as the faecal sludge was observed to be thick.

4.1.1. Risk of Heavy Metal in Agricultural Soils

In most cases heavy metal contamination of soils is due to release of into the environment from activities such mining, animal manure, petrochemical spillages, paints and wastewater treatment sludge application to soils [Wuana & Okieimen, {2011)]. Heavy metals commonly found at contaminated sites include the following; lead (Pb), chromium (Cr), copper (Cu), zinc (Zn), cadmium (Cd), arsenic (As), nickel (Ni) and mercury (Hg), [Wuana & Okieimen, (2011)] to name just a few. These metals do not undergo microbial or chemical degradation as it is the case with organic contaminants which are oxidized to carbon (IV) oxide by microbial action. The heavy metals also inhibit biodegradation of organic compounds [Wuana & Okieimen, (2011)].

The most hazardous to humans among heavy metals in sludge are cadmium, mercury, and lead, while copper, zinc, chromium, and nickel. In high concentrations these metals are particularly poisonous to plants [Hashem, {2000)]. The data from the two areas of Mogoditshane Table 3 and Table 4 show average heavy metal content in faecal sludge generally low as a potential risk to humans. The results however, show escalated amounts of copper compared to the South African limits for Spreading of sludge on fields. However, pH has an effect on the behaviours of these metal Metals are bound to soils at a pH exceeding 6.5 and/or with a high organic matter content. If the pH is below this value, if organic matter is consumed or if all feasible soil adsorption sites are saturated, metals become mobile and can be absorbed by crops and contaminate water bodies [Wuana & Okieimen, {2011)]. The FS sample results shown in Fig. 4 show that, the sludge pH ranges between 7 and 8. This indicates that, this pH could increase the binding behavior of metals in soils once the sludge is applied for agriculture.

An important consideration in the application of sludge to farmland is its limitation of possible addition of toxic elements and any beneficial effects are secondary to this. This is because crops can accumulate toxic elements from sludge-amended soils and where heavily contaminated sludges and excessive rates of application are used plants may accumulate concentrations which are toxic to plants. Among the heavy metals in the analysed sludge, the most hazardous ones to humans are cadmium and lead. However, copper, zinc, chromium, and nickel in high concentrations are particularly poisonous to plants [Reid, {2014)].

Table 3

Faecal Sludge Heavy Metal concentrations for 25 Mogoditshane Pits
metals	Mogoditshane metals (mg/kg)
	layer 1				layer 2				Limits for Spreading on field
	Mean	max	min	SD	Mean	max	min	SD	South Africa	European Union
Ni	8.62	15.84	0.32	0.33	9.33	11.32	6.70	0.31	200	300–400
Cr	28.72	55.41	12.15	2.07	39.63	69.45	13.16	1.91	1750	1000–1500
Pb	9.31	16.69	4.13	0.42	8.29	15.65	3.19	0.35	50.5	750–1200
Zn	390.59*	880.80	137.63	5.23	327.55	573.00	150.85	6.19	353.5	2500–4000
As	1.68	6.68	0.03	0.14	3.82	9.17	0.79	0.31	15	-
Cd	0.32	0.82	0.13	0.02	0.39	1.04	0.11	0.06	15.7	20–40
Cu	40.36	72.02*	22.04	0.35	43.03	62.35	29.36	0.38	50.5	1000–1750
Mn	199.83	255.86	150.69	7.01	204.07	301.01	151.06	8.77	-	-
Fe	8483.00	15294.71	3389.51	421.17	11854.66	20447.08	4058.08	557.76	-	-
Ca	15278.11	40579.42	2.48	2.15	3254.28	21813.19	2.49	2.13	-	-
Sn	1.79	2.69	1.25	0.14	2.15	5.87	1.08	0.46	-	-
Na	2114.77	5127.31	741.11	53.97	2254.72	3538.46	1296.92	114.95	-	-
Mg	3406.30	4157.74	2512.98	163.29	2831.97	3825.28	1896.00	137.44	-	-
K	3950.40	3950.40	3950.40	52.79	3863.72	3950.40	3343.66	74.50	-	-
*exceed the South African limits for spreading on fields. - No limits

Heavy Metal concentrations in FS is at acceptable levels compared to South African (SA) and European Union (EU) standards for spreading in the fields, except for Cu which exceeds SA limits [Commission Regulation (EU) 2023/915 (2023)]. It is observed though that the SA limits for Cu are a lot more stringent compared to the EU limit for the same [Dikinya et al., (2011)]. The Cu levels in Table 3 by far exceed SA limits in all pit levels. However, these values are below EU limits. This indicate that, although both higher in Broadhurst results than the results for both Mogoditshane and Broadhurst areas, the risk posed by the presence of heavy metals in soils, is generally low in FS as compared to what is normally found in sewage treatment sludge [Nowrouzi, and Pourkhabbaz (2014)]. This suggests that the land application of treated FS is much more viable than the land application treated sewage sludge. However, environmental concerns regarding land disposal such surface-water and groundwater pollution and transmission of human and animal diseases should never be taken lightly.

While soil characterization would provide an insight into heavy metal speciation and bioavailability, attempt at remediation of heavy metal contaminated soils would entail knowledge of the source of contamination, basic chemistry, and environmental and associated health effects (risks) of these heavy metals. In Tables 3 and 4a range of heavy metal concentrations were found as 0.03–20,447 mg/kg and 0.04–1.513 mg/kg for Mogoditshane and Broadhurst, respectively. For all the two study areas, the concentrations are in the order of Cd ˃˃As ˃˃Sn ˃˃Ni Pb˃˃ Cr ˃˃Cu˃˃ Mn˃˃ Zn ˃˃Fe. Although heavy metals present in pit sludges tested, their concentrations are not posing any risk especially to plants. Therefore, the presence of heavy metals in FS is not significant when compared to their presence in Sewerage treatment Sludge (STS) [Tytła M., 2019]. In that case, the application of treated FS to agricultural land should be encouraged rather than the application of STS to agricultural soils.

Sewage sludge is known for its composition rich in organic matter (OM) and essential biogenic compounds, particularly nitrogen (N) and phosphorus (P), crucial for promoting plant growth [Hammer & Hammer, 2014; Wang et al., 1999; Hashem, 2000]. Nevertheless, it also contains heavy metals, including toxic ones such as cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), nickel (Ni), lead (Pb), and zinc (Zn) [Gimeno-García et al., 1996; Malkki, 1999)]. The presence of these heavy metals in sewage sludge implies that, depending on their concentration and duration of exposure, they can potentially pose environmental and health hazards, primarily because of their capacity to bioaccumulate within the food chain [Zhang et al., 2010].

The primary sources of heavy metals in sewage sludge encompass domestic and industrial wastewater discharges, sewerage system corrosion, and runoff from urbanized regions or roadways [Wuana & Okieimen, 2011]. In light of the elevated heavy metal concentrations found in sewage treatment sludge, the application of treated faecal sludge (FS) for enhancing food production warrants serious consideration. This approach not only mitigates concerns related to co-treatment of FS with wastewater, which can lead to operational issues at wastewater treatment plants (WTPs), but also presents a more sustainable and business-savvy solution for FS management, ultimately contributing to improved food security.

The significant nutrient content observed in FS samples from our study area positions it as an excellent candidate for land application in agriculture. However, it's important to be mindful of potential runoff issues in areas near surface water bodies where FS has been applied, as the high nutrient content could contribute to eutrophication problems. To mitigate these impacts, the application of sludge should be coupled with best ploughing practices (BPP) to enhance soil retention and prevent nutrient runoff. It's reassuring to note that, in accordance with international standards, the concentrations of heavy metals in FS remain within acceptable limits, further endorsing the feasibility of using FS in agriculture.

Table 4

Faecal Sludge Heavy Metal concentrations for 25 Broadhurst Pits relative to the South Africa and European Union Limits for Spreading on Agriculture
Metals	Broadhurst metals (mg/kg)
	layer 1				layer 2				layer 3				layer 4				Limits for Spreading on field
	Mean	max	min	SD	Mean	max	min	SD	Mean	max	min	SD	Mean	max	min	SD	South Africa	European Union
Ni	3.34	6.18	0.28	0.08	2.02	3.35	1.13	0.05	1.88	4.00	0.13	0.04	2.53	3.54	1.52	0.78	200	300–400
Cr	13.43	20.95	10.68	0.60	10.78	13.44	6.58	0.42	15.15	22.51	8.12	0.69	8.65	11.57	5.73	3.94	1750	1000–1500
As	0.83	1.88	0.33	0.14	6.32	27.22	0.45	0.30	0.87	1.72	-0.04	0.19	7.28	13.79	0.77	3.28	15	-
Cd	0.19	0.69	0.00	0.01	0.30	0.69	0.00	0.02	1.26	5.64	0.00	0.02	0.96	1.82	0.10	0.96	15.7	20–40
Cu*	591.61*	1100.99	242.00	37.72	672.15*	1031.44	397.18*	49.76	733.17*	1513.44	307.23	30.67	455.08*	562.50	347.65	26.21	50.5	1000–1750
Sn	0.64	1.34	0.35	0.11	1.47	3.94	0.18	0.43	1.03	2.71	0.36	0.24	11.69	22.70	0.67	4.76	-	-
*exceed the South African limits for spreading on fields. - No limits

4.2 Geo-accumulation Index Calculation

The Geo-accumulation Index (Igeo) was calculated for the heavy metals analyzed in this study, and the results are presented in Table 5 and Fig. 6. The Igeo values provide insights into the potential ecological risk associated with these heavy metals. For nickel (Ni), chromium (Cr), and arsenic (As), the Igeo Index values were found to be below zero, indicating a negligible risk of environmental contamination by these elements. Specifically, Ni, Cr, and As exhibited negative Igeo values across all layers (L1, L2, L3, and L4), emphasizing their minimal impact on environmental pollution.

In the case of chromium (Cr), there was a slight indication of potential pollution within layers L3 and L4, as suggested by the Igeo values slightly below zero. However, this risk was still considered low and may not significantly affect the environment. Conversely, copper (Cu) exhibited Igeo values above zero, indicating a moderate risk of pollution. These values were consistently below 2, signifying that while Cu poses some risk, it remains within manageable limits.

Figure 5 illustrates the Geo-accumulation Index values of these heavy metals across different sludge layers, providing a visual representation of their ecological implications.

Table 5

Geo-Accumulation Index for Heavy Metals
Metal	B_n	layer 1		layer 2		layer 3		layer 4
Metal	B_n	C_n	I_geo	C_n	I_geo	C_n	I_geo	C_n	I_geo
Ni	22	3.34	-0.694	2.02	-0.912	1.88	-0.943	2.53	-0.814
Cr	100	13.43	-0.747	10.78	-0.842	15.15	-0.695	8.65	-0.938
As	13	0.83	-1.070	6.32	-0.188	0.87	-1.049	7.28	-0.127
Cd	0.3	0.19	-0.073	0.3	0.125	1.26	0.748	0.96	0.630
Cu*	50	591.61	1.198	672.15	1.253	733.17	1.291	455.08	1.084
Sn	2	0.64	-2.228	1.47	-1.029	1.03	-1.545	11.69	1.958

Overall, the Geo-accumulation Index calculations emphasize that the heavy metal content in pit latrine sludge, especially for Ni, Cr, and As, does not pose substantial environmental risks. However, continued monitoring and proper management practices are advisable to mitigate potential pollution concerns, particularly in the case of copper (Cu).

This paper offers a fresh perspective on faecal sludge management, emphasizing its potential benefits and environmental concerns in a specific geographical and agricultural context. The findings, therefore, cater to the local context of Botswana, where water scarcity and agriculture are critical issues, making the research relevant and novel in addressing these challenges. The study concludes that faecal sludge is a valuable resource rich in organic matter and essential nutrients for plant growth. However, the high concentrations of nitrate and phosphate in pit latrine sludge pose a significant risk of groundwater pollution. The presence of heavy metals, while generally low, requires careful management to avoid environmental contamination. Based on the findings, the following conclusions and recommendations are provided:

Faecal sludge contains high levels of organic matter, nitrate, and phosphate, making it a potential fertilizer for plant production. However, elevated concentrations of nitrate and phosphate in pit latrine sludge pose a serious risk of groundwater pollution, emphasizing the need for improved management practices. The concentration of heavy metals in faecal sludge samples are generally low, with copper levels exceeding South African limits but remaining below European Union limits and the Geo-accumulation Index calculations indicate minimal environmental risk for nickel, chromium, and arsenic, but a moderate risk for copper suggesting a need for careful monitoring and management. Based on the findings of this research, the following recommendations are suggested:

Provide enhanced support for faecal sludge management practices in peri-urban and rural areas to minimize environmental and health risks.
Develop and implement guidelines and best practices for the safe and controlled use of faecal sludge as a fertilizer, taking into account its nutrient content.
Continue monitoring heavy metal concentrations in faecal sludge and implement measures to mitigate potential environmental pollution.
Launch awareness campaigns to educate communities about the risks associated with improper faecal sludge disposal and the benefits of adopting sustainable practices.

Acknowledgments: This work was made possible through the generous support of The Water Research Commission (SA) KSA11:K5/2297/11. . Special thanks go to Ditebogo Nage, and Able Keitseng for the laboratory assistance they offered in this project.

Author Contributions: Phillimon Odirile Conceived and designed the experiments; Veronica Obuseng designed the experiments, analyzed the data and contributed in the writing of the paper; Mohau Moshoeshoe and Lamong Tshwenyego collected data, performed the experiments, analyzed the data and also contributed in the writing of the manuscript.

Funding Author: Author Dr Phillimon Odirile has received Funding from Water Research Commission (South Africa) # KSA11:K5/2297/11.

Data availability; Data is provided within the manuscript.

Conflicts of Interest: The authors declare no conflict of interest.

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No competing interests reported.

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Assessment of Faecal Sludge Quality, Heavy Metal Pollution, and Ecological Risk: Implications for Sustainable Agriculture

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Abstract

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1. Introduction

1.1 Research Context and Novelty

2. Study Area

3. Materials and Methods

3.1. Sludge sample Collection and Preparation

3.2. Analytical Methods

3.2.1. Metal Analysis by ICP-OEP

3.3 Data Analysis

4. Results and Discussions

4.1. Nutrient Content in FS

4.1.1. Risk of Heavy Metal in Agricultural Soils

4.2 Geo-accumulation Index Calculation

5. Conclusion and Recommendations

Declarations

References

Additional Declarations

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Version 1