The efficiency of advanced alkaline disinfection in closed systems has been demonstrated to depend on the balance of ammonia concentration, pH, exposure time, temperature, total solids content, post-treatment storage time and mixing effectiveness. When the alkaline system is a closed one, an additional pathogen stressor is added: pressure. The effect of the alkaline dosing has been assessed for raw, aerobically, and anaerobically digested biosolids that produce un-ionized ammonia at concentrations of 0.05% to 2%. Overall, the addition of pressure as a factor in disinfection increased the inactivation of Ascaris egg to Class A levels by 50 to 99%, at the temperature range of 40 – 55oC. The systems studied were compared to an alkaline process operated under open conditions, which limited the concentrations of ammonia available due to the Henry’s gas law. Under a closed pressurized system, the impact of un-ionized gaseous ammonia was greatly increased and the resulting time required for inactivation reduced from hours/days to minutes. Also, the use of ferrate in alkaline treatment is an emerging technology that has recently gained interest due to a new synthesis method, making it a more cost effective option. In the next few years, it is expected that alkaline disinfection of biosolids will be optimized in relation to the factors stated above, at much lower doses of the alkaline agents. The closed-system alkaline processes that will be developed will be more energy-efficient, cost-effective, and have full control of potential odorous emissions.
Major developments have been made with regard to alkaline stabilization and disinfection over the past two decades. Current research is being directed toward advanced disinfection at lower doses of the alkaline agent and the assessment of long-term stability (Reimers et al, 2004; Brewster et al, 2004; Abu-Orf et al, 2003). With the use of alkaline disinfection for wastewater and sludge treatment, problems arise such as odors, time constraints, and stability issues. Without the optimization of all controllable conditions in alkaline systems, these problems will continue. Most alkaline processes are open systems (i.e. the reactor is open to atmosphere rather than air-tight) in which temperature is more difficult to control, and factors such as increased pressure or bactericidal action of un-ionized ammonia are not present to aid in disinfection. Open systems require significantly higher temperature levels, consequently higher alkali doses, e.g. 0.5 to 1 kg quicklime (CaO)/kg DS, than those that would be adequate in closed systems. They also may require a longer retention time in the reactor vessels to achieve adequate disinfection. A closed system offers a number of combinations, for example: long reaction time and very low alkali dose; higher dose and shorter react time; an optimized dose at a higher pressure.
The usage of ferrates under alkaline conditions can also yield a disinfected, stabilized residual. The resulting pH of the treated biosolids is around 12 to 13 and the disinfected product is a stabilized residual as a result of resulting from the oxidant’s high ORP and affinity to react with sulfides, mercapitan and alkyl amines. In addition, ferrate would be an ideal stabilization agent for unstabilized or partially stabilized biosolids since ferrate reacts with the degradable organics along with the above odiferous compounds. Nutrient control results from the initial high pH, causing calcium phosphate precipitation, and then as the pH drops below 9, iron oxides adsorb the negatively charged monohydrogen phosphate.
The following will present observations in the disinfection of sludges in such closed systems where, due to lack of gas exchange with the atmosphere, the duration of the disinfection process and its effectiveness are vastly improved at much lower doses of the alkaline agent. We will also elucidate the usage of ferrates in the management of biosolids with respect to disinfection and stabilization as well as the control of nutrients and metals.
BACKGROUND OF ALKALINE STABILIZATION
The use of lime as a disinfectant for wastewater solids dates back over 2000 years to the Roman Empire where it was used to treat “night soil” (Dean and Smith, 1974). In 1740 the first large-scale water treatment system was inaugurated in Paris. Twenty-two years later in 1762, England established the first waste treatment system based on chemical precipitation that utilized lime (Votorantim Cimentos, 2003). The use of lime continued into the 19th century in Europe and Great Britain. Also during this time period, the use of other alkaline agents such as kiln dust and ashes were discovered to be as effective as lime for stabilization and disinfection and began an active role in the United States around 1915. The use of lime is also noted with regard to its simplistic treatment of sewage in military camps (Backman, 1942). In the 1930’s and 40’s processes were developed and patented such as the Putnam process, which utilized lime, carbon and ferric chloride, and the Lewis process, which consisted of a mixture of lime, wood pulp and Cottrell dust (Francis, 1992). In an article in Public Works in 1935, the plants that utilized these new technologies were being discontinued due to “no improvement by the addition of either paper pulp or clays, the only effect noted being a tendency toward the more rapid clogging of the filter bed” (Streander and Blew, 1935). The next major developments in lime stabilization/disinfection did not occur until the 1960’s.
Developments in the 1960’s and 70’s by Doyle and then Farrell led the way in the development of the scientific parameters associated with lime use in relation to the disinfection and stabilization of sludges. Doyle’s research delineated the need for achieving pH 12 and holding it for 2 hours to achieve sufficient pathogen reduction. This determination was the basis of the Class B pathogen reduction criteria in the 503 regulations (Doyle, 1967).
In the 1980’s, the standard method of lime use in wastewater treatment was the addition of lime slurry to liquid sludge. As alternatives to this process were investigated, a study by Westphal and Christensen compared these new alternatives to the standard practice.
The study examined the use of iron and lime conditioning followed by vacuum filtration dewatering and the combination of sludge cake and dry quicklime. The determination was made that both process modifications are as effective as the standard process in reducing fecal coliform and fecal streptococcus densities, provided the EPA criteria of pH >12 for 2 hours is met. Also to be noted, all three lime stabilization processes performed as well as or better than mesophilic aerobic digestion, anaerobic digestion, and mesophilic composting in reducing the densities of fecal coliform and fecal streptococcus (Westphal and Christensen, 1982).
Lime dosage is a major factor in assuring adequate pH, temperature and therefore disinfection of the sludges. In alkaline processes, heat is generated by the reaction of lime with the water in the sludge resulting in heat evolution at 15,300 calories per gram mol or 488 BTU per pound (Gutschick and Lewis, 1982).