PREVENTION OF MARINE EUTROPHICATION THROUGH PHOSPHORUS RECOVERY FROM SEWAGE SLUDGE INCINERATION ASH AND SCWO, SUPER CRITICAL WATER OXIDATION, RESIDUES.
Sustainable handling of municipal organic waste and sewage sludge has as an important goal to recycle resources without supply of harmful substances to humans or the environment (Levlin 1999, Hultman and Levlin, 1997). Another important goal is to avoid to deposit waste and sludge on landfill. In Sweden a tax of 250 SEK/ton on all deposited solid waste was introduced year 2000 (SFS 1999:673). Deposition of incinerable waste has been prohibited and in year 2005 there will be a ban on deposition of all organic material on landfill (SFS 2001:512). Incineration (ATV, 1997) and SCWO, Super Critical Water Oxidation (Gidner et al., 2000), are methods that eliminates all organic content and the potential energy of the organic material can be utilised. SCWO occurs in water of a supercritical phase at a temperature above 374 °C and a pressure higher than 22 Mpa. Sludge incineration requires that the sludge has to be dried to 40 % dry substance. The energy produced from the incineration is consumed by the drying and there is therefore no net energy recovery from sludge incineration. With use of SCWO, energy can be recovered also from sewage sludge and organic waste with too low dry substance for incineration. Anaerobic digestion eliminates half of the organic content and half of the energy can be utilised as methane gas. Increasing the energy recovery from sewage sludge and organic waste decreases the need of fossil fuel and makes the society more sustainable. Handling of sludge and organic waste can be integrated by use of food waste disposers and transporting the organic waste to the wastewater treatment plant with the sewer net (Karlberg and Norin, 1999). The resources in sludge and solid organic waste remaining after SCWO or incineration are nutrients such as phosphorus. Development of method to recover phosphorus will make handling of municipal organic waste and sewage sludge more sustainable.
Since phosphorus is needed as a fertilizer in the agriculture, a requirement for getting sustainable wastewater treatment is to create method to recover the phosphorus from the wastewater. Most of the phosphorus used in the agriculture originates from mining of phosphate ores which thus is a limited resource. The global deposits of economically mineable phosphate are estimated to be 109 ton phosphorus and the total amount phosphorus in the sediments is estimated to be 1015 ton (Butcher et al., 1994). Many different phosphate minerals are available, but only apatite (calcium phosphate, Ca3(PO4)2) is used for phosphate production (Corbridge, 1995). Phosphate can be economically produced by leaching apatite mineral with sulphuric acid (McKetta and Cunningham 1990). In 1995 the world phosphate rock production was 160 000 ton per year (as P2O5), having tripled over the last 40 years. About 90% of this amount is used as fertiliser. At this rate of consumption the known apatite reserves have been estimated to last for a period up to 1000 years. However, if the present increase in world population and the increasing need for fertiliser for food production is taken into account, the supply of phosphate may well be crucial within a century. Apatite ore is thereby the limited resource that must be preserved by phosphate recovery.
Phosphorus removal from municipal wastewater has a long tradition in Sweden to protect receiving waters from eutrophication with the first plant for phosphorus removal in Åker municipality in 1961. Chemical precipitation was the only economically feasible technology known at that time and future implementation of phosphorus removal in Sweden has mainly been based on chemical precipitation with iron salts as dominating precipitation agent. Only a few plants are operated with biological phosphorus removal in Sweden, although this process technology is widely used internationally.
Recently, Swedish policy requires phosphorus to be recycled and, because agricultural sewage sludge re-use is increasingly limited, this is putting pressure on cities to develop phosphorus recovery systems. In a number of cases, authorisation to construct sludge incinerators is being given condition that phosphorus must be recovered. A national goal has been proposed to the Swedish government that at least 75% of phosphorus from wastewater and other biological wastes should be recovered at latest by 2010 without risks for health and environment. The Swedish Environment Protection Agency (SEPA) has been given a commission from the government to better evaluate possibilities to implement this goal and propose modifications. A draft "state-of-the-art" report on phosphorus recovery has been worked out (Balmér et al., 2002). Phosphorus recovery by spreading sludge on agricultural land has been stopped to a high extent due to resistance from the farmers´ organisation (LRF) and food industries and disposal of organic material to landfill will be forbidden in 2005. The pollution content in sewage sludge (Levlin et al, 2001) makes direct use of sludge as fertilizer difficult. An alternative to phosphorus recovery is to pure sludge from toxic contaminants (Levlin et al, 1996). However, phosphorus recovery reduces the amount of materials and thereby the transport cost.
Precipitation of phosphorus into the sewage sludge decreases emission of phosphorus from wastewater treatment plants. However, phosphorus precipitated in sludge can be released at sludge disposal (Rydin, 1996) and thus contribute to marine eutrophication. Release of phosphorus to the marine environment favours growth of nitrogen fixation algae, thus decreasing the effect of reducing nitrogen emissions (Naturvårdsverket, 2003). Recovery of phosphorus from sludge and organic waste thereby reduces phosphorus release from disposal sites and thereby the risk for marine eutrophication.
Leaching with acid and base is a promising method for phosphorus recovery from waste water treatment sewage sludge. Phosphorus recovery from SCWO, Super Critical Water Oxidation, residues by leaching with acid and base in order to recover phosphorus from sludge has been studied in experimental works (Hultman et al, 2002, Hultman and Löwén, 2001, Levlin et al., 2002, 2004a, 2004b, Stark, 2002, Stark and Hultman, 2003) at the department of Land and Water resources Engineering, KTH, by a research group with Bengt Hultman (professor), Erik Levlin, Kristina Stark (doctor student) and Monica Löwén (laboratory staff). Samples of ash taken from the co-incineration of sludge with municipal waste, sludge incineration plant and SCWO residues have been leached with acid, hydrochloric acid and base, sodium hydroxide. The metal and phosphorus content before leaching has been analyzed as well as the metal and phosphorus content in the leachate.Leaching with acid gives a higher release of phosphorus compared to leaching with base. The largest degree of leached phosphorus (80 - 100 % at acid concentrations below 0,5 M) was obtained by leaching SCWO residue with acid. Acid leaching of sludge incineration ash gave 75 – 90 % leached phosphorus at the concentration 1 M. Alkaline leaching of sludge incineration ash and SCWO-residue gave 50 – 70 % leached phosphorus at the concentration 1 M. When leaching with base, the calcium content in the sludge binds phosphorus as calcium phosphate. On leaching with acid it is difficult to recover phosphorus as other products than iron phosphate. However, iron phosphate has no commercial value as raw material for the phosphate industry, and the low solubility makes it less favourable to use as fertilizer. For the production methods used in phosphate industry is iron phosphate a pollution, which binds phosphorus and reduces the amount of produced phosphorus.
PROJECT OBJECTIVES AND METHODOLOGY
The project will focus on evaluating phosphorus leaching from deposition of residues from the phosphorus recovery methods examined in earlier research, leaching sewage sludge incineration ash and SCWO, Super Critical Water Oxidation, residues with acid and base. Alkaline leaching has a promising potential for recovery, since a pure non-iron phosphate phosphorus product can be obtained. However, the lower degree of recovery compared to acid leaching makes the risk of phosphorus leaching from deposition of the residues larger. The evaluation will be complemented by leaching experiments on residues from phosphorus recovery, equivalent to those made by Rydin (1996).
Project leader will be Erik Levlin and Monica Löwén will perform the laeching tests. The project will be in performed co-operation with the doctor student project by Kristina Stark, which is financed by the MISTRA programme "Urban Water". Bengt Hultman who 2003 was appointed to professor in Environmental Engineering, is leader of the Research Group for Water Engineering and Resource management at the department of Land and Water resources Engineering, KTH, and will assist as scientific adviser.
The project can start in June 2004. The laboratory leaching test will be performed during year 2005. The results will be evaluated and presented in Sweden and internationally during year 2006.
The research will be also in co-operation with external parts such as municipalities, SEPA, Swedish Water Companies etc and internationally with organisations as CEEP and the Swedish- Polish co-operation project.
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