Heat-dried sludge compared to sludge dried on sand beds is free of pathogens and weed seeds and is therefore safer to use. However, under normal conditions, heat-dried sludge is more powdery and more difficult to spread and mix with soil than conventional sludge. It is initially repellent to water although once it becomes partially moist, it readily absorbs more water. Therefore, heat-dried sludge must be further treated before being sold as a fertilizer.
It sells in 50- to 65-lb sacks. Although Milorganite, due to its guaranteed 6% nitrogen content, (coming from Milwaukee's beer waste) is still in demand, a similar product made in Chicago (only 31/2 to 4% nitrogen) is not as popular, and only a small percentage of their current supply is being sold.
The destruction of pathogens during heat drying is such that only 2 coliform bacteria/gr remain in over 100 samples. Bacterial, parasitic, and viral enteric pathogens commonly found in sewage have the same order of heat sensitivity as coliforms.
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