Can a precast pit latrine concrete floor withstand emptying operations? An investigation from Malawi

For fecal sludge from households in low- and middle-income countries to be treated offsite it needs to be removed, which can be greatly affected by the pit latrine floor design. However, it is unclear whether precast pit latrine concrete floors (latrine slabs) can withstand emptiers and their equipment. To investigate this issue, 28 prefabricated latrine slabs were purchased in two cities of Malawi. They were first visually evaluated, and then their compression strength was tested. Additionally, each seller was asked a series of questions to better understand their business, training, and construction practices. Results showed that households should perform due diligence to ensure that they are purchasing a safe precast latrine slab. Commonly reported problems included nonstandard reinforcement material and spacing, in addition to slabs that were not thick enough or were not large enough in diameter. The results of this research illustrate the inherent complexity in ensuring high-quality decentralized sanitation solutionsand how one component, the user interface, if implemented poorly, can affect the rest of the value chain. The findings from this work can help inform training and initiatives that engage artisans and suppliers who play a role in the provision of onsite sanitation service delivery.


INTRODUCTION
Many households in low-and middle-income countries currently use and will continue to use pit latrines, the contents of which need to be treated either onsite or offsite. The purpose of the pit latrine floor is to cover the pit, effectively separating and containing the fecal sludge while also supporting self and imposed load; this structure is the first For fecal sludge to be treated offsite, it needs to be removed from the pit. This process can be affected greatly if the pit latrine floor was not constructed correctly. Researchers estimate a typical adult mass of 61 kg (Walpole et al.  Ten readings using the same hammer were performed for each slab in the vertically downward position. Thereafter, a reinforcement material verification exercise was conducted by manually crushing the slab with a sledgehammer ( Figure 1). The size and spacing of the reinforcement were recorded as were the materials used in their construction and whether the reinforcement materials were corroded.
The results were compared to technical specifications by Harvey () and WEDC ().
Ethical approval for this research was obtained from the Malawi Government, National Commission for Science and Technology (study protocol number P02/18/255).

RESULTS AND DISCUSSION
The results of this study provide guidance in terms of what options should be recommended to improve household sanitation facilities in low-and middle-income countries. These facilities ultimately require fecal sludge to be treated offsite while there is also an important role promoting private sector sanitation service providers selling these products.
In Lilongwe, no sellers had dome slabs; only three dome slabs could be found in Mzuzu.

Slab dimensions
Earlier work in Mzuzu found a mean pit diameter below Only three of the slabs (3/28) had a cutout for a ventilation pipe.

Presence of reinforcement
Each seller indicated during their interview that their slab (28/28) contained reinforcement. None of the slabs had reinforcement exposed or not fully covered with concrete at the time of purchase. While all of the flat slabs (25/25) had some form of reinforcement, it was observed to be of variable material, size, and spacing (Table 1). Notably, instead of steel reinforcing bars, four slabs had used scrap metal (such as reusing a window frame of a car and a sofa spring), and some of this scrap material was observed to be corroded (Figure 1). Two other flat slabs only used binding wire for reinforcement. In one case, the slab mostly had the proper size of reinforcement, but one length of reinforcement was substituted with a sofa spring. None of the flat slabs met the reinforcement recommended by Harvey ().

Compressive strength
The compression strength between slab samples is a strong indicator of durability. Each seller (28/28) reported that the slabs had been cast at least 21 days prior to data collection. Each of the slab surfaces was troweled. Only one slab had a compression strength >20 N/mm 2 , the minimum applicable curve for the rebound hammer used in this study, which is an indication that the recommended concrete mix proportions were used. When the researchers constructed a flat slab based on the quality standards of Harvey () and WEDC (), the compressive strength was found to be >20 N/mm 2 , which indicates that if constructed properly, the slabs in this study should be able to achieve this threshold compressive strength. For slab construction, a ratio of cement to sand to gravel of 1:2:4 or 1:3:6 is recommended (Harvey ). Some sellers did not know the ratios used in construction, but when available it was reported that the ratios ranged from 1:5:1 to 1:1:2. No seller reported the use of either of the recommended ratios.

Cost
Many independent variables affect the selling price of slabs, including taxes, labor, and transportation (whether the seller has their own transport to collect materials or uses public transport). The current minimum wage in Malawi is MK25,000 (USD$35) per month. When researchers constructed a flat slab based on the quality standards of Harvey () and WEDC (), the cost was MK26,311 (USD$37) (materials MK15,630; transport MK8,300; 2 days labor MK2,381). The cost of each slab in this study was even lower. A provider who builds slabs that meet quality standards may not be able to compete with the prices of inferior but aesthetically pleasing products on the market.

Seller businesses
It was more common for sellers to have received formal training in slab construction in Mzuzu (4/5 sellers) than in Lilongwe (2/10 sellers). The duration of training was reported to be 1 week and was mostly overseen by nongovernmental organizations. The reported slab production per seller was 2 to 20 slabs per week, indicating a relatively small-scale business. Many sellers (10/15) had been in business for more than three years.
It was observed by researchers in this study that the delivery of precast latrine slabs from the seller to the household is challenging and expensive (MK2,000 to load the slabs into the buyer's vehicle and an extra MK5,000 to transport the slabs). The use of local public transport for a slab is difficult; for example, consider the task of lifting a slab onto the top rack of a local minibus. Only two (of 28) slabs had lifting inserts. Rahman et al. () reported that in Malaysia, prefabricated toilet construction allows promotion by sellers of proprietary designs, cost savings through mass factory production, and testing in advance under a controlled factory environment; however, none of these factors were observed among the sellers in this study.

Study limitations
It was not possible to determine the exact mixture or age of the concrete, but researchers estimated by visual observation that the studied slabs were all older than 28 days. Despite the negative results of this small study, the fact that latrine slabs are on the local market indicates that someone is buying them. Each seller has a reputation to maintain and would be motivated to improve their quality control if they lost sales as a consequence of low quality.
However, how can a seller compete if they follow quality standards that would raise the cost of making a slab above the price of their competitors? In addition, there are no published reports of local accidents occurring while using or emptying latrines with precast slabs versus those made onsite.
As the need for fecal sludge to be treated offsite increases, prescriptive specifications are even more important. National guidelines based on a local context and proper strategies should be implemented to monitor and evaluate precast latrine slabs for their safety during emptying operations and by users. The findings from this study can be used to inform training and initiatives that engage artisans and suppliers that play a role in the provision of onsite sanitation service delivery.