The sanitary biogas unit
Sanitary Biogas Units are installations where the gasplants have been built in order to treat the waste of latrines. Human faeces are the main digestion material. Additional feeding with animal dung or kitchen waste is possible. Hygienic latrines have to fulfill the following requirements:
-no handling of human excrete by man; even accidental touch shouId be avoided
-no access of flies to undigested excrete
-no worms may escape from the latrine pit
-no bad odour and no indecent appearance
Important design criteria concerning hygiene and construction quality must be observed. Main planning criteria are the expected sanitary conditions which depend on frequency of use, frequency of cleaning, and safe slurry disposal. Slurry should be used for fertilizing trees or shrubs but not vegetables. The slurry may also drain into a soak pit. Energy and manure provision are of lesser importance but should be optimized whenever possible.
Construction of Toilets
Toilets connected to a simple fixed dome biogas plant should be latrines where a minimum of water is used for cleaning. Flushing toilets are not suitable for connection to biogas plants of less than 30 m3 digester volume because of the danger of diluting the slurry and thus reducing the retention time.
The toilet chamber is connected to a vent pipe which passes the roof. It is placed outside, if possible not shaded, and is painted black as to heat up for better draft.
Fig.32: Construction details of toilets
The toilet floor (a) has a groove serving as the toilet pan. The highest slurry level (b) and the lowest slurry level (c) depend on the gas storage requirements of the biogas plant. There are a few but very important details to be observed: (1) The inlet consists of a piece of pipe of 6" diameter placed absolutely vertical in order to avoid soiling the sides. (2) The bottom rim of the inlet piece is separated from the lower system in order to prevent worms from crawling out of the toilet. (3) The inlet piece ends always above the highest slurry level. (4) Below the inlet piece is a chamber of larger surface in order to avoid floating feces piling up in the pipe. Feces should drop directly into the slurry but never on parts of the structure which are normally above the slurry level. On the other hand, the dropping chamber should be as small as possible in order to release fresh feces as quickly as possible into the biogas plant. This is important for avoiding bad odour and for producing the biogas there, where it can be collected and utilized, which is the inside of the dome. (5) The down pipe is straight and at least of the same diameter as the inlet piece (6). A vent pipe passes above the roof (7)