Pig manure has been found to be the main contributor to stream pollution in Hong Kong, as most of the manure is discharged into the watercourses without any treatment. As a solution to the pig manure problem, the pig-on-litter (POL) system was initiated in 1987.  Also known as in-situ composting, pigs n this system are raised in pens, the floor of which is covered with a 30 cm thick layer of sawdust (bedding material), mixed with bacterial product to aid decomposition.  A 10-month study was carried out to study the specific role of bacterial product on the performance of the POL system.  The findings of these experiments revealed that the success of the system depends on proper management of the system.  It was also found that this in-situ composting process relies on the establishment and maintenance of suitable microorganisms.  This means that the pig manure itself is the source of microbes, and a proper combination of sawdust bedding with pig manure provides the necessary environment for these microbes to propagate.

 The spent pig litter disposed from the POL system contains high concentrations of organic matter and nutrients, which can be re-utilized as a fertilizer and/or conditioner, but requires further composting in windrows to reach full maturity.  The time required for the spent pig litter varied with the operation strategies used in the composting process.  This study identified the best operation strategies for efficient composting of spent pig litter.  Thus, less frequent turning was observed to decrease the rate of composting and it took longer time to reach maturity.  On the other hand, moisture content in the range of 50-60% appeared to be suitable for an efficient composting.  Seasonal temperature also affected the rate of composting.  Composting during winter resulted in a lower temperature peak in the piles and thus a slower decomposition rate.  To combat this, the frequency of turning can be decreased such that a satisfactory thermophilic temperature is achieved.

 Further composting of the spent pig litter in windrows was found to reduced the phytotoxic effects of the immature spent litter compost since it converted the biodegradable organic matter to a more stabilized product.  The availability of the chemical factors, which affect phytotoxicity of the spent pig litter, was also significantly reduced during composting.  Further composting of the spent pig litter also eliminated the pathogen Salmonella.  Results of this study revealed that a temperature of around 60-65oC for 2-3 weeks is enough to kill the pathogen Salmonella.