The value of composting excreta with dry organic matter is discussed in Annex 1. Composting toilets are of two types: those such as double-vault latrines, which use anaerobic bacteria, and continuous composting latrines, which make use of aerobic bacteria.
Each latrine consists of two chambers or vaults used alternately (Fig. 6.32). Initially a layer of about 100 mm of absorbent organic material such as dry earth is put in the bottom of one vault, which is then used for defecation. After each use, the faeces are covered with wood ash or similar material to deodorize the decomposing faeces and soak up excess moisture.
Fig. 6.32. Double-vault latrine
When the vault is three-quarters full, the contents are levelled with a stick and the vault is completely filled with dry powdered earth. The squat hole is then sealed. While the contents of the first vault are decomposing anaerobically, the second vault is used. When the second tank is full, the first one is emptied through a door near the bottom and the chamber is reused. The contents may be used as a soil conditioner.
Each vault should be large enough to hold at least two years' accumulation of wastes so that most pathogenic organisms die off before the compost is removed. Recommended vault sizes range from 1.1 m3 (Winblad & Kalama, 1985) to 2.23m3 (Wagner & Lanoix, 1958).
Normally the superstructure is built over both vaults, with a squat hole over each vault. A cover sealed with lime mortar or clay should be fitted in the squat hole above the chamber not in use. A flyproof lid should be placed on the other hole when it is not being used for defecation. Flyproof vent pipes may be provided to avoid odour nuisance in the latrine, although covering the faeces with ash is reported to be sufficient to eliminate bad smells.
Control of the moisture content is vital for proper operation of the latrine. Consequently composting latrines are not appropriate where water is used for anal cleaning. It is usual to collect urine separately, dilute it with 3-6 parts of water and use it as a fertilizer (although this may cause a health hazard). Some latrines are constructed with soakpits below the vaults so that excess moisture can drain into the ground (Fig. 6.33). This allows for the disposal of urine into the vaults but with consequent loss of a valuable fertilizer and possible pollution of the groundwater. Wood ash, straw, sawdust, grass cuttings, vegetable wastes and other organic material must be put into vaults to control moisture content and improve the quality of the final compost.
Fig. 6.33. Double-vault latrine with soakpits
Besides providing a reusable resource, the double-vault latrine has the added advantage that it can be built anywhere. Since the vault contents are kept dry, there is no pollution of the surrounding ground, even if the vault is buried. In rocky areas or where the water table is high the vaults may be built above ground. Walls and base should be watertight.
Double-vault composting latrines have been successfully used in Viet Nam (McMichael, 1976) and Guatemala (Buren et al., 1984). When tried elsewhere they have usually been unsatisfactory. Most of the disadvantages revolve round the problem of controlling the moisture content. Proper operation of the latrine is difficult to understand and considerable effort may be required to educate local people in its use. The contents are often allowed to become too wet, making the vault difficult to empty and malodorous.
Continuous composting toilets
These consist of watertight sloping chambers about 3 m in length. Excreta fall into the chamber from a toilet. Dry organic kitchen and garden waste is tipped in through a separate opening (Fig. 6.34).
Fig. 6.34. Continuous composting toilet (A)
Fig. 6.34. Continuous composting toilet (B)
Inverted U-shaped ducts and a ventilation pipe encourage the passage of air through the mass, preventing it from becoming anaerobic and allowing excess moisture to evaporate. As new material enters at the top of the chamber, older material gradually moves to the bottom and then slides into a smaller compartment from which it is removed periodically.
Such toilets have proved satisfactory in holiday homes and other isolated buildings in industrialized countries, where they are sometimes installed in a cellar beneath the latrine and kitchen.
Attempts have been made in Botswana and the United Republic of Tanzania to adapt the design to suit African materials and customs (Winblad & Kalama, 1985) using tanks made with concrete or sand and cement blocks. They were found to be inappropriate because of their high cost and sensitivity to user operation. Retaining the proper carbon-nitrogen balance and moisture content is crucial to proper operation. In practice, it has been found that moisture content is the most difficult to control. Fly and odour problems are also common, particularly soon after commissioning.