PROJECT DESCRIPTION
BACKGROUND
The deposition of sludge and ash creates major environmental problems in Sweden and other parts of the EU. With the production of electricity and heat from wood increasing rapidly, the amount of ash being produced is also increasing. If this ash is not recycled, then the longterm production potential of the forests cannot be sustained. Sludge from sewage treatment works has a water content of 70-80%. Mechanical reduction of the water content below this level is impossible. Although previous attempts to pelletise sludge have failed, a new technique, the Kl-method, has been developed in which the pellets are continously sprayed with lime powder and/or ash. This causes the pellets to stiffen whereafter they can be cut to appropriate lengths. This new technique of pelletising sludge has many advantages. The final product contains all the nutrients and organic materials, but the unpleasant smell is removed and the product is sterile. The dry pellets are as easy to spread as commercial fertilisers. The pellets are also storable and can be used for dispersal in forests. Therefore, this project will either result in the returning of nutrients and humus of sludge and ash to the natural cycle, or alternatively, in the retrieval of their energy content or in safer deposition.
OBJECTIVES
The aim of the project was to recycle sewage sludge and ash from biomass fuel, by means of a full-scale prototype plant for directly pelletising sludge and ash. The sludge and ash were to be subsequently used as fertilizers in forestry and/or agriculture. Where the quality of the sludge meant that it was not appropriate for spreading, energy recovery or safer deposition would be facilitated. With the construction of a prototype plant, it would be possible to disseminate this new technique to other sewage treatment works and heating plants in Europe and in other parts of the world. The long-term objective was to solve the problems of sludge and ash treatment in the County of Västerbotten. It was anticipated that nearby municipalities would transport their sludge to Lycksele for pelletising. The prototype plant would therefore be adapted to facilitate future expansion. The Lycksele prototype plant would also serve as an example and source of inspiration for other municipalities considering building similar plants. The project would therefore also include marketing and promotional activities. Locally produced, environmentally branded fertilizer for agriculture and forestry would enable the recycling of the nutrients in sludge and ash for use as a fertilizer. Before the commencement of the project, discussions with ASSI Domän were initiated concerning their financial participation in the longterm testing of the fertiliser on land owned by them. Different financial applications were to be submitted to the SLU, who would be assisting in the implementation of the tests. The main tasks in the action plan were: · Technical and environmental assessment · Ash treatment · Prototype plant for pelletising sludge and ash · Development of a system for transport and storage · Investigation and environmental impact analyses Initially, it was expected that the plant would recycle 2,200 tons of sludge (TS 20-25%) and 700 tons of ash (from a biofuel powered thermal power stations) per annum.
RESULTS
Impact on the environment One of the consequences of intensive forestry combined with the deposition of ash is that the forest soil becomes depleted of basic cations, such as Ca2+, K+, Na+, Mg2++. Also, this depletion is accelerated by acidification. When removing forest fuel and timber, nitrogen is also removed, and deposition of nitrogen is low in northern Sweden. However, nitrogen can be added to forest soil after timber cutting (time periods and doses in accordance with the Local Forest Conservation Board’s guidelines). The distribution of ash has been tested for a long time. In 1997, The Local Forest Conser-vation Board circulated a comprehensive description of the environmental consequences, presenting different research results. Controlled recycling of ashes has been deemed feasible without causing a negative environmental impact (however, recycling of ash only has not been deemed to have any positive impact on the growth of the forest – except on peaty soil). The heavy metal content in sludge from the sewage treatment plant falls significantly below Swedish limit values. Sludge from Swedish sewage treatment plants is, with a few exceptions, of good quality with a high intermix of humus substances (approx. 50% of TS). During the project the sludge in Lycksele was shown to meet all the requirements for sludge to be spread on arable land (i.e. for production of food). Suppliers of ash to the Lycksele plant tested their material for heavy metals and caesium. Regular tests of the end product were also carried out at the plant. If at any time the heavy metal content exceeded the allowable margin, the pellets were not spread on lands but were instead utilised as raw material for fuel, or were deposited. According to investigations, the energy value of sludge pellets was estimated to be approx. 3,000 kWh per tonne TS (comparable to the energy value of wood-chips, which is also 3,000 kWh per tonne TS). This has made combustion possible at a water content as high as 40-50 per cent, which has reduced the consumption of energy in the manufacture of pellets for fuel purposes. The project also achieved a more efficient use of natural resources, as the product has partly replaced artificial fertilizers. The recovery of 2,200 tonnes of Lycksele’s sludge leads to an annual production of approx. 1,200 tonnes of “environmentally-friendly” fertilizer. In terms of nitogen content, this contains amounts equivalent to approx. 60-100 tonnes of artificial fertilizer. Pellets produced during the initial phase of the project were used to perform a number of fertilization and combustion trials. AssiDomän, one of the land owners, designated land for the distribution of pellets west of Lycksele urban district. The area contained 55-year old pinewood. Permission to distribute pellets was applied for and granted by the Local Forest Conservation Board, on certain conditions. Owners and users (reindeer breeding) of the grounds had either approved of the distribution or had no objections. SLU (The Swedish University of Agricultural Science) established a field test to evalu-ate the impact on the production of wood biomass and on the environment of the distribution of pellets manufactured from sewage sludge mixed with wood ashes from the prototype plant. The assignment included investigation of the following: · The chemical structure of the pellets produced · The impact of pellets fertiliser on the growth rate of trees and on the environ-ment during two vegetation periods (2001-2002) Pellets were distributed during 2002 over younger thinning areas in various doses (150 – 600 kg/hectare). Testing equipment was placed in the area, trees were measured, marked, and analyzed. Specimens of soil water, needles and humus coating were also analysed. The pellets and their constituent materials, such as sludge and ash, were also analysed for Cu, Pb, Cd, Cr, Ni, Nh4, NO3, organic substances, Ph , % Carbon, and total % nitrogen. Initial result of the distribution of pellets in growing forests were obtained, but it was also found that additional follow-up is required. The product appeared to contain small quantities of heavy metals, which were largely supplied by the ash. The total nitrogen content was low, and organic substances accounted for about 45 per cent. The pH of the product amounted to 8.0. The low content of nitrogen requires large doses to be distributed to achieve the best possible effect. The results so far are outlined for below. · Structure of the pellets. Based on the contents of various substances in ash and sludge, the content of ash in the pellets is estimated to be approximately 40 percent of the overall weight. Ash from Skellefteå has a relatively high content of heavy metals, in particular chromium, copper and lead. The high content of ash means that the nitrogen content is relatively low, and this is compounded by a loss of nitrogen caused by the intermixture of ash. A positive element of the ash is the addition of basic cations derived from Ca, Mg and K, and Boron. · Enrichment of heavy metals in shrews. With regard to toxic heavy metals, the analyses did not indicate any difference at all in the livers or kidneys of shrews, when comparing treated and untreated material. Bioaccumulation of toxic heavy metals in shrews could not be demonstrated three months after treatment; Shrews have been shown to be sensitive heavy metal ”detectors”. · Needle analyses. Higher concentrations of nitrogen in needles after fertilising could be established only in mineral-fertilised areas. In pellets-fertilised areas needles indicated a somewhat higher content of aluminium, boron, cobalt, chromium and po-tassium than other areas. · Soil water analyses. The chromium content in soil water was not higher in pellets-fertilised areas than in control areas. However, the content of other heavy metals in pellet-fertilised areas was generally higher than in control areas. However, in several cases the highest content of heavy metals, such as cadmium, cobalt, nickel, lead and zinc, was found after the test on 9 October. With this innovative technique of pelletising sludge many environmental advantages are achieved. The final product contains all the nutrients and organic materials, but the unpleasant smell is removed and the product is sterile. The dry pellets are as easy to spread as commercial fertilizers. The pellets are storable without any inconvenient smell or environmental problems. If ash is added to the sludge, pellets will be especially suitable for dispersal in forests. If the sludge contains e.g. heavy metals, the palletising can facilitate energy recovery in incineration plants or the development of safer deposition methods. The project will therefore result in the returning of nutrients and humus of sludge and ash to the natural cycle.