**1. Introduction**

Faecal sludge management (FSM) in most developing countries of the sub-Saharan region is ineffective and insufficient, which causes a deepening of sanitation problems [1–4]. Improper pit emptying and sludge disposal have been attributed to factors such as shortages in suitable sanitation, poor drainage systems, and high groundwater fluctuations [1,5]. Further, sludge management is impacted by high transport and disposal costs in landfills. The permanent airspace disposals can also lead to human and environmental impacts [6]. Previous and recent latrine building projects have focused on constructing latrines without considering the emptying process and sludge management strategies [2].

Sanitation challenges within sub-Saharan Africa have led to recurring epidemics of sanitation-related diseases, including soil-transmitted helminth infections [7]. Outbreaks can occur periodically where water supplies and sanitation provisions are inadequate, most

**Citation:** Mamera, M.; van Tol, J.J.; Aghoghovwia, M.P.; Nhantumbo, A.B.J.C.; Chabala, L.M.; Cambule, A.; Chalwe, H.; Mufume, J.C.; Rafael, R.B.A. Potential Use of Biochar in Pit Latrines as a Faecal Sludge Management Strategy to Reduce Water Resource Contaminations: A Review. *Appl. Sci.* **2021**, *11*, 11772. https://doi.org/10.3390/ app112411772

Academic Editor: Amanda Laca Pérez

Received: 6 October 2021 Accepted: 3 November 2021 Published: 13 December 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

frequently in the developing world [3,4,8,9]. Between 1970 and 2011, African countries reported over 3 million suspected cholera cases, representing 46% of all cases reported globally [8]. Sub-Saharan Africa accounted for 86% of reported cases and 99% of deaths worldwide in 2011 [9–11]. Statistics of this nature are alarming and need urgent redress. While the reasons for these conditions are complex, part of the problem is the difficulty in accessing clean water and safe potable water, lack of sanitation, and the high costs involved. Pollution problems from pit latrines depend on climatic conditions, geological formations and soilscapes on the rate of soil contaminants migration. These factors lead to a need for scientific assessment of sludge management and pollution challenges. This can ensure that these sanitations are properly sited, designed, installed, monitored, and maintained [3,12]. Although the use of pit latrines as compared to open defecation can be beneficial, there are still concerns that they may cause dreadful human and ecological health impacts. This is associated with microbiological and chemical contamination of drinking water supplies through leaching into groundwater and surface water [11].

Integrated approaches for access and improvement of sanitation and water are needed to address these issues to curb the potential danger to public health and the environment. Creating simple and sustainable solutions for managing human excreta plays a direct role in slowing down the rate of environmental damage. This can be done by seeking alternative means that aim at reducing environmental pollution by faecal sludge, while not further depleting severely limited freshwater resources. Incorporation of soil a conditioner such as biochar has a high impact on the reduction in contaminant leaching [13–17].

Biochar is a high carbon-rich adsorbent produced from any organic biomass at high temperatures in conditions with limited oxygen [18]. Many studies to date have mostly focused on the potential of biochar to improve soil fertility for agricultural uses [19–21]. There are, however, numerous prospective benefits of integrating biochar in FSM studies. Such merits include: micro-organic mitigations [22]; reducing malodour [23]; contaminant barrier (bacteria and heavy metals) [15,24,25]; reduction in nitrogen [26], and carbon dioxide losses [27].

A gap in knowledge regarding the use of biochar to reduce the environmental threat of faecal sludge still exists. In the recent past, the potential of biochar to reduce leaching has been recognized, and several studies have been conducted on organic and inorganic pollution restriction by biochar. This review aimed to evaluate the potential of biochar in FSM through literature, which focused on biochar, sanitation, and faecal sludge studies. This review merits attention, because it explores alternative means for faecal sludge management, which can also be implemented in developing countries such as Mozambique, South Africa, and Zambia to minimize seepage of pit latrine waters and provide a sustainable soil conditioner for crop production.

## **2. On-Site Sanitation Systems**

On-site sanitation is characterized by treatment and disposal of human waste, which is not removed to an off-site sanitation system [28,29]. Such sanitation facilities store wastes at the site of disposal, which decompose in situ [30]. These systems have two main categories; the wet latrines, which use water for flushing, and the dry latrines, which function without water sources. The different types of on-site sanitation systems [28,30,31] are pit latrines, ventilated improved pit latrines (VIPs), urine diversion (UD) toilets (Figure 1), ecological sanitation (EcoSan) latrines, Fossa Alterna, anaerobic biogas reactors, and septic tanks. A common pit latrine is composed of a simple top structure constructed over a pit and collects waste [32]. Improved pit latrines are a simple and low-cost type of sanitation system [13].

**Figure 1.** (**a**) Examples of on-site sanitation designs [30]; (**b**) typical structure for a VIP toilet system in South Africa [31] reproduced from the reference, copyright 2001, CC-BY-4.0.
