FIELD OF THE INVENTION

The invention relates to a filter granule for use in the purification of wastewater, to a method of manufacture for the filter granule and to the use of a filter provided with the filter granule. BACKGROUND OF THE INVENTION

Prior art in the field of the invention has been disclosed in Finnish patent publication FI113246B. It discloses a method for the manufacture of a granular filter material, wherein a raw material of quicklime and/or dolomite and gypsum are mixed together with the effect that the mixture undergoes an exothermic reaction in the presence of water. The resulting mixture is worked into granules for use in the purification of grey water, i.e. mainly to remove phosphorus compounds from grey wastewater.

In the prior art, products manufactured from limestone are generally used in the treatment of wastewater. Limestone is not a renewable raw material, so it is preferred to use by-products and waste to substitute for lime. The problem in the prior art is its functionality and applicability with grey wastewater only. However, ,in most of the wastewater systems in rural areas the different types of wastewater have not been separated. Thus, the system according to patent

FI113246B is not applicable for most of the wastewater purification sites in rural areas.

Currently, the purification efficiency required of the purification of black wastewater in rural areas is over 80% for organic matter, over 30% for nitrogen and over 70% for phosphorus. The purification methods used roughly include two principal methods: chemical precipitation or sand filtering. An old type of wastewater system often includes 2-3 concrete tanks to precipitate any solid material. The problem in such tanks is their poor efficiency to purify phosphorus.

Some of the small-scale purification plants used to substitute for old systems include precipitation tanks for organic material and chemical precipitation in the last tank. An alternative is to build a sand filter plant to extend the old tanks. This is not often possible due to rocky or excessively clayey soil. In ad- dition, sand filtering must often be renewed between 10-15 years. The current purification methods used to substitute for or supplement the old systems are expensive investments. In peat production areas the leaching of solid material in harvest areas is controlled first in field ditches by sludge sumps and headland pipes and then by regulation barriers and sedimentation basins in water- collecting ditches. Humus and phosphorus dissolved in the water are effectively removed by means of overland flow fields. The use of overland flow fields used in peat production areas is not always possible due to the terrain. In case it is not possible to arrange an overland flow field, an alternative is for example chemical purification. The chemical precipitation of phosphorus and humus generally requires active feeding equipment .

Solid matter and phosphorus are leached from drainage areas in peatland forests particularly during the years following the drainage. Water emissions particu- larly in terms of solid material are controlled by sedimentation sumps, digging breaks and overland flow fields. In areas where digging breaks and overland flow fields are not possible there is a risk of the leaching of solid material and nutrients . The organic solid matter in the runoff from drainage areas is rich in nutrients, so the leaching of solid material increases eutrophication. During flood peaks, the flow of solid material and thereby the water emissions are at the highest.

Another problem is often the acidity of the leachates from drainage areas. The most acidic water is found on the so-called earth of alum close to the coast and in the slate areas of Eastern Finland. This is because the earth's sulfides form sulfuric acid when provided in an oxygenous state. In arable land this problem is generally controlled by regulation French drains to regulate the surface of groundwater so that the sub- water part is not oxidized.

Both functioning and closed mines release acidic water to the environment. The acidity results from the concentration of metals or from the oxidation of sulfide in sulfide minerals into sulfate by the effect of air, which forms sulfuric acid. Cost-effective neutralization of water by passive neutralizing solutions is particularly important in closed mines. The invention introduces an alternative to the use of non-renewable limestone in the neutralization of minewater . In many aspects, this is extremely important because the availability of lime is reduced in the long run due to the depletion of lime mines. OBJECTIVE OF THE INVENTION The objective of the invention is to remedy the above- mentioned defects concerning wastewater, leachate and the purification thereof. In particular, it is the objective of the invention to disclose a novel filter granule, a method of manufacture therefor and use thereof to allow affordable and versatile possibilities for the purification of different types of wastewater and leachate.

SUMMARY OF THE INVENTION

The granulation of the filter granule according to the invention for use in the purification of wastewater and leachate is made using as a raw material the ash produced in the combustion of biomasses and granulated by means of water. The most suitable granular size for the filter granule is 3-15 mm, for example approximately 5-10 mm, depending on the stage and object of purification. Even larger grain sizes are also possible in applications where the use of same granules is long lasting.

The combustion of biomasses produces ash including agents that precipitate phosphorus and humus. In addition, ash neutralizes acidic water. The invention removes phosphorus and solid materials from wastewater. The invention is very well suited for the treatment of acidic water because it also removes acidity from the water. Even though the bioash contains small amounts of heavy metals, the detrimental elements are chemically tightly bound to the ash, so they are not dissolved in the water being purified to a detrimental extent. The solubility of detrimental elements is re- duced during granulation. Ash granules granulated from suitable mixtures of ash are very stable and useful in the purification of wastewater.

The invention also relates to a method for the manu- facture of a granular filter granule. In the invention water is added to ash produced in the combustion of biomasses and the mass is mixed and granulated in a rotating treatment, such as in a rotating drum or on a rotating tray. For the ash to be readily processable, the dusty ash must be granulated. The rotating fine mass of ash turns into granules when it achieves suitable moisture, for example in the range of 25-35 w-%. The optimum moisture depends of the properties of the ash mixture .

After formation of the granule, yet another layer, i.e. a coating, may be granulated to the surface of the fresh granule with a thickness of for example 1-2 mm using suitable ashes or a mixture of ash and a pre- cipitation chemical and/or neutralizing capacity improving agent. The phosphorus precipitating chemical in the coating may be 0-30 % Fe(II)S0 or Al ₂ 0 ₃ . The neutralizing capacity improving agent may be for example 0-50 % lime or steel slag.

The filter granule is produced by using a mixture of fully burnt ashes wherein the amounts of base cations, iron and aluminum are high but the amounts of phosphorus and heavy metals are as low as possible. The ash granule is coated with a mixture of ash and ferric sulfate (3%v/W) .

The invention also relates to a filter including a cartridge for installation in a wastewater tank, the lower end of the cartridge being provided in flow communication with the waste tank. The upper end of the cartridge is provided in flow communication with the outlet pipe from the waste tank and the cartridge is filled with the filter granule according to the invention. As the lower end of the cartridge is substan- tially open and its upper end is provided in flow communication with the outlet pipe, the wastewater entering the waste tank steadily flows in an upward direction via the lower part of the tank through the cartridge, so that the filter granules in the cartridge are able to act in a purifying fashion on the wastewater flowing through.

If necessary, the cartridge can be easily lifted and emptied and filled with new filter granules, but in a preferred embodiment of the invention the lower end of the filter is provided with a release or another structure to open the lower end of the cartridge and allow the dropping of the filter granules down to the bottom of the waste tank. Thus, the used filter granules can be removed from the waste tank by the same suction truck which is used to empty the waste tank. Further, the services provided by the suction truck may _% also include the supply of new filter granules, so that the cleaning of the waste tank, removal of the used filter granules and installation of the new cartridges or filter granules are always carried out during the same visit. The invention provides a competitive and eco-efficient alternative to update old tanks in compliance with the current requirements and an alternative for the current systems based on chemical precipitation and filtering.

The operation of the filter granule according to the invention has been tested under experimental condi- tions with two experiments where 89% and 97% of the phosphatic phosphorus in the water flowing through the filter cartridge were bound to the filter granules. In experiments carried out under real conditions for the tank purification efficiency an amount of 93% was obtained for the total removed phosphorus with a load from 8 persons during three months .

The invention also relates to an ash filter for the purification and neutralization of leachate in drainage areas, peat production areas and in the mining in- dustry. According to the invention the ash-based filter granule according to the invention is used as the filter structure in the filter, which filter granule is used to build a suitable barrier or filter field through which the water being purified is conveyed. In case the main purpose of use is the neutralization of water, the surface layer of the ash granule may be granulated with a mixture of lime or steel slag and ash. The transportation of the filter mass and the logistics in the drainage areas and peat production areas can be made cost-effective if the transportation package can be utilized as barrier structures. In case a bulk bag is used as the transportation package and two of its sides are made of a durable meshwork, the bag will permeate water and work itself as a barrier structure. Thus, the transportation of material and building of the barrier can be made by the same equipment. The bags may be provided with a step that is im- permeable to water in the inflow direction so that some of the solid material drifting in the water are sedimented in front of the bag. Thus, the bag also functions as a regulation barrier, i.e. it evens out the flood peaks and prevents excessive drying of the ditches particularly in sulfate areas. In case it is important to even out the flood peaks, the bags can also be used in field ditches to slow down the flow of water to the water collecting ditches. The bag may also be used as a bridge in drainage areas to facilitate moving in the drainage areas .

In case the bag is used on the terrain for a long time, the lifting lugs may become weak and break when lifted. The sides of the bag may be provided with pipes which are secured to the lifting lugs. Thus, new lifting ropes can be fed through the pipes. In addition, the pipes seal the bag against the edges of the ditch. In easily erodible soils with high flow rate the bag may be attached to a mounting plate pressed into the ground. The plate prevents the erosion of soil around the bag and guides all flow of water through the bag.

In peat production areas and forest drainage areas the content of the bag can be spread in the immediate sur- roundings as a soil conditioner after use. The ash produced in the combustion of biomass decisively improves the nutrient situation both in forestry peat- lands and peat production areas that are no longer in use .

A filter field can be built following a free sedimentation basin or sump in the ditch or for the final portion thereof. The water discharging from the sump moves through the filter mass. The field can be used in the case where the amount of water being neutralized is large, such as at mines. The field structure may also be so disposed that the water is conveyed over or under the field and it also flows through the ash granule mass to some extent in the vertical direc- tion. The barrier or field according to the invention may be so constructed that any debris and some of the solid material in the water are first filtered by coarser- fractioned ash, whereafter a finer-structured ash fraction is used to carry out the actual neutralization of acidity and chemical precipitation of phosphorus and humus. Another option is that a ferrous and aluminous ash granule is used to carry out the chemical removal of phosphorus and humus in a first stage and a second stage includes the neutralization of water by an ash granule having a high neutralizing capacity. The staging of purification may be provided in a single bag, bag by bag or for each segment of the filter field.

The present invention and the different embodiments thereof have considerable advantages as compared with the prior art. The basis of the invention is ash produced in the combustion of biomasses, i.e. a pure and renewable natural product. According to the invention the ash produced as a by-product of energy production can be used in the granular form in the purification of different types of water. The invention is extremely well suited for the purification of all and not just grey wastewater. Further, purifying chemicals can be easily added to the surface layer of the granules produced from the ash to adsorb phosphorus and organic materials on the surface of the granules or flock and precipitate impurities when the chemicals are dis- solved in the water being purified. Further, materials or chemicals increasing the acidity neutralizing capacity of the filter mass may be used in the surface layer. It is also noteworthy that no waste is produced by the use of the method according to the invention; instead, the used granules may be used as such in the fertilization of soil such as a forest. Below, the invention will be described in detail with reference to the accompanying drawings, in which

Fig. 1 illustrates one embodiment of the invention and Fig. 2 illustrates a second embodiment of the invention .

Fig. 1 illustrates a wastewater tank 1 having an inlet pipe 2 through which the wastewater flows into the tank and an outlet pipe 3 via which the wastewater is able to flow out of the tank. In addition, the wastewater tank 1 has a cartridge 5 filled with the filter granule 4 according to the invention. At the lower end the cartridge is mainly open and at the up- per end it is closed. In addition, the outlet pipe 3 extends from its closed · upper end. Thus, the wastewater entering the tank is able to flow down in the tank and from the bottom in an upward direction via the bottom of the cartridge through the filter granules towards the opening of the outlet pipe 3.

This way, the wastewater entering the tank does not interfere with the operation of the filter; instead, the water being filtered steadily and slowly flows up in the cartridge through the entire area of the car- tridge, so that all of the filter granules effectively take part in the purification of wastewater before the water flows out of the tank.

Further, the lower end of the cartridge 5, i.e. the bottom 6 thereof, is provided with a release 7 to open the bottom so that the granules 4 inside the cartridge are dropped to the bottom of the tank. This makes the use of the tank simpler because the used granules can be sucked out of the tank while it is being emptied with a suction truck. Then, the bottom of the empty cartridge can be locked back on and refilled with new granules .

Fig. 2 illustrates a second embodiment of the inven- tion wherein a ditch 8 is provided with a bulk bag 9 filled with the filter granules according to the invention. The flexible bag adapts to the edges of the ditch so that it constitutes a relatively sealing barrier structure. Thus, the surface 10 of water is able to rise upstream of the barrier and the water flows through the filter granules in the bag. The filter granules effectively remove different kinds of impurities from the flowing water. The area of the bottom of the bulk bag can also be made less permeable to water or completely impermeable to water to be able to use the bag to regulate the flow of water in the ditch and also to make better user of the entire volume of the filter granules in the bag as a filter. The replacement of the granules is easy just by lifting a new bag to the ditch next to the prior one and removing the used bag from the ditch. The filter granules in the used bag make good fertilizer to be spread in a forest or on fields.

The invention is not limited merely to the examples referred to above; instead, many variations are possible within the scope of the inventive idea defined by the claims .