Caustic Soda Treatment is a cost-effective chemical treatment for faecal sludge from pits and trenches. It
uses caustic soda also known as lye (sodium hydroxide: NaOH) as an additive to create a highly alkaline environment and thereby sanitises sludge from human waste. It significantly reduces the public and environmental health risks of latrine sludge.Mixture of solids and liquids, containing mostly excreta and water, in combination with sand, grit, metals, trash and/or various chemical compounds. A distinction can be made between faecal sludge and wastewater sludge. Faecal sludge comes from on-site sanitation technologies, i.e. it has not been transported through a sewer. It can be raw or partially digested, a slurry or semisolid, and results from the collection and storage/treatment of excreta or blackwater, with or without greywater. Wastewater sludge (also referred to as sewage sludge) originates from sewer-based wastewater collection and (semi-)centralised treatment processes. The sludge composition will determine the type of treatment that is required and the end-use possibilities.Describes technologies for on-site collection, storage, and sometimes (pre-) treatment of the products generated at the user interface. The treatment provided by these technologies is often a function of storage and is usually passive (i.e. requires no energy input), except a few emerging technologies where additives are needed. Thus, products that are ‘treated’ by these technologies often require subsequent treatment before use and/or disposal. In the technology overview graphic, this functional group is subdivided into the two subgroups: “Collection/Storage” and “(Pre-)Treatment”. This allows a further classification for each of the listed technologies with regard to their function: collection and storage, (pre-) treatment only or both.Refers to the methods through which products are returned to the environment, either as useful resources or reduced-risk materials. Some products can also be cycled back into a system (e.g. by using treated greywater for flushing).A functional group is a grouping of technologies that have similar functions. The compendium proposes five different functional groups from which technologies can be chosen to build a sanitation system:
User interface (U), Collection and Storage/Treatment (S), Conveyance (C), (Semi-) Centralised Treatment (T), Use and/or Disposal (U).
A sanitation system is a multi-step process in which sanitation products such as human excreta and wastewater are managed from the point of generation to the point of use or ultimate disposal. It is a context-specific series of technologies and services for the management of these sanitation products, i.e. for their collection, containment, transport, treatment, transformation, use or disposal. A sanitation system comprises functional groups of technologies that can be selected according to context. By selecting technologies from each applicable functional group, considering the incoming and outgoing products, and the suitability of the technologies in a particular context, a logical, modular sanitation system can be designed. A sanitation system also includes the management and operation and maintenance (O & M) required to ensure that the system functions safely and sustainably. The utilisation of products derived from a sanitation system.
A sanitation system in which excreta and wastewater are collected and stored or treated on the plot where they are generated.
The means of safely collecting and hygienically disposing of excreta and liquid
wastes for the protection of public health and the preservation of the quality of public water bodies and, more generally, of the environment.

Waste matter that is transported through the sewer.
An open channel or closed pipe used to convey sewage. See C.3 and C.4
Used water from any combination of domestic, industrial, commercial or agricultural activities, surface runoff/stormwater, and any sewer inflow/infiltration.

Caustic soda is a white, alkaline, odourless material supplied as flakes packed in drums. It is used to increase the pH of blackwater or sludge and create a highly alkaline environment that destroys pathogens. The optimum dosage to reach the recommended pH of 12 is around 26 g of soda per litre of faecal sludge. The exact amount, however, depends on the characteristics of blackwater or sludge. Its effect can be enhanced by ensuring complete mixing, a longer contact time and a higher dosage of caustic soda. The pH should be maintained above pH 12 for a minimum of 2 hours to ensure an adequate reduction of pathogens. The Caustic Soda Treatment process should be undertaken as a batch process and can be used to treat both solid and liquid sludge. After treatment, pH decrease towards neutral usually within 24 hours. After neutralisation, the supernatant can be pumped off and safely infiltrated into a Soak Pit D.10 . Care should be taken in areas with high a groundwater table as the supernatant still contains nitrogen and phosphorous which can pollute water bodies. The treated solid fraction at the bottom may be applied as a soil amendment or dried and used as cover for sanitary landfills.

Design Considerations

Caustic Soda Treatment can either take place above ground in a separate tank or below ground. In areas with a high groundwater level or in flood prone areas it is recommended to always use above ground tanks. Separate tanks may be needed for the preparation of the soda solution slurry and for the post-neutralisation of the treated effluent respectively.

Materials

Caustic Soda Treatment needs a reactor vessel that can either be an above ground tank (between 1–30 m3) or a pit below ground with tarpaulin lining. An additional smaller container is needed for the preparation of the caustic soda solution (e.g. 200 L plastic drum). For an even distribution of caustic soda in the tank it is mixed into the sludge either manually or using a mixing pump. The type of pump required depends on the consistency of the sludge. A separate pump is needed for removing the treated effluent from the tank and a shovel or vacuum pump for the removal of solid material. In addition a water testing kit (particularly for pH, E.coli, total suspended solids and turbidity) is needed as well as personal protective equipment (PPE) including a mask, gloves, boots, an apron or safety suit. A steady supply of caustic soda is also required.

Applicability

Caustic Soda Treatment is particularly suitable for the rapid response phase due to its short treatment time, simple process and use of readily available materials. With trained and skilled staff, it allows for a safe, cost-effective and extremely fast treatment of faecal sludge.

Operation and Maintenance

Caustic Soda is corrosive due to its high alkalinity, therefore a regular maintenance of pumps is required. During storage, caustic soda must be kept dry at all times because it absorbs and reacts with water. Due to potential health risks when handling caustic soda (see below) skilled and trained personnel must follow respective health and safety protocols and wear proper PPE.

Health and Safety

Caustic Soda is corrosive to the skin, eyes and lungs. Adequate PPE must be worn when handling it to prevent irritation to eyes, skin, respiratory system, and gastrointestinal tract. The occupational Exposure limit for caustic soda is 2 mg per cubic meter for a 15-minute reference period. Washing with cold water is recommended for affected skin and eye areas followed by rinsing with borax-boric acid buffer solution. Medical attention should be sought. Protection from fire and moisture must be ensured. Caustic soda is an alkaline material which reacts strongly with moisture. Trained personnel must follow health and safety protocols.

Costs

Caustic Soda Treatment is a relatively cheap treatment option. In general, caustic soda is twice as expensive on the market as lime S.17 . Costs may vary depending on the availability and costs of local materials and chemicals/soda. As part of a proper health risk management, costs for PPE and respective trainings for staff need to be considered.

Social Considerations

Proper health and safety protocols should be in place and include the provision of PPE and respective trainings for involved staff.