Whenever there is a need to reuse back the sludge for farming purposes, usually government regulations would require that a certain level of disinfection needs to be carried out for the sludge on top of the conditioning and stabilization processes. Strict regulations require that this critical step be applied for the protection of public health especially at places where plots of farms are located very near to residential areas and source of drinking water. This is to prevent harmful pathogens from spreading to the general public which can lead to critical health concerns.
There are different ways and options to achieve sludge disinfection and mainly what makes one process different from the other is basically the cost aspect. The most common techniques applied are pasteurization, heat applied processes like drying, incineration and pyrolysis, long term storage in liquid digested form, lime treatment with pH higher than 12, composting, addition of chlorine and high energy radiation technology using beta and gamma rays. Some options such as the lime treatment other than stabilizing the sludge can also play a part to disinfect it as well but for some like the aerobic digestion (excluding thermophilic digestion) will only go up to the extent of reducing the number of pathogens. The most commonly used methods measured in terms of cost and effectiveness and are widely adopted in most countries especially in Europe are the pasteurization or using the long term storage of dewatered aerobic and anaerobic sludge.
Sludge pasteurization is usually carried out under low temperature up to 70degC for about 30 minutes. There are two approaches to achieve this; one is by applying steam via means of direct injection and another using indirect heat exchange. Between those two, direct steam injection seems to be a more feasible approach as use of heat exchangers will often lead to scaling, fouling and especially clogging caused by buildup of organic matter over time. A typical sludge pasteurization system using the direct steam injection approach usually involves feeding a preheated sludge going up to 55degC and then mixed with incoming steam before diverting it to reactors and maintained for specific time period. Currently in terms of cost management, the option here will only be feasible provided that there is continuous supply of sludge and it will only work for those plants with high capacity treatment per day. In order to ensure efficient heat transfer so that operating cost can further be lowered, in-line mixing has to be incorporated into the design which also helps to maintain a uniform heating.
Long Term Storage
Sludge disinfected using this option requires that earthen lagoons are used and most important is the wide area on the availability of land. Site identified must not be close to residential areas as odor generation will become a nuisance and groundwater monitoring well needs to be installed in order to gauge the extent of percolation. One weakness about this method is that it requires long detention period of time and for those systems which generate continuous supply of sludge, consideration has to be put in to deal with the bulk of storage problem. However, a clear advantage when looking at another angle is that, sludge that are contained and kept meant for land farming can be taken out only when it is needed. Long term storage usually require that it must be stored for least minimum 60days if the temperature is at 20degC while in colder climate conditions this will require that the sludge to be stored for even longer period.