FIELD OF THE INVENTION

The invention relates to a method for disinfecting sludge and to a soil amendment and a fertilizer produced by the method.

BACKGROUND OF THE INVENTION

Treatment of sludge is ^' one of the most expensive steps in the purification of wastewater. The amount of sludge is great and it is insanitary due to pathogenic microbes it contains. The recirculation of nutrients contained in the sludge, e.g. in agriculture, has only been possible to a limited degree for technical, economic and hygienic reasons.

As EC legislation has come into effect, the utilization of sludge in farming has practically completely stopped in Finland. The price increase of fertilizers in recent years has restored the interest of farmers in sludge as a soil amendment, but strict hygiene standards have led to a situation where the sludge has mostly been taken to landfills and only a small portion of the sludge has been used in bioreac- tors for the production of biogas.

Some plants use a so-called lime stabilization method to make sludge hygienic. In lime stabilization, either burnt lime (CaO) or slaked lime, i.e. calcium hydroxide (Ca(0H ₂ )), is added to sludge. The pH of sludge must rise to at least 12 and its temperature must climb to 55C for two hours. Lime stabilization is very expensive in investment and operating costs. The removal of gas which is required to remove, ammonia produced in the stabilization in order to avoid odor annoyances adds to the costs of the method.

Anaerobic digestion of sludge to produce biogas■ (CH ₄ ) has become more common at medium-sized and large wastewater plants. The costs of this method are also high, although the gas produced is a valuable source of energy for the wastewater plant. The outlet sludge from the plant must nevertheless be made further hygienic before any possible utilization.

Other treatment methods include aerobic composting and disposal at a landfill.

The methods described above are expensive, they are accompanied by odor and other environmental nuisances, and the utilization of sludge is often not possible.

OBJECTIVE OF THE INVENTION

The objective of the invention is to disclose a new type of microbiologically effective and economi- cally advantageous method for disinfecting sludge. Specifically, the objective of the invention is to alleviate the above-mentioned problems.

Further, the objective of the invention is to disclose a soil amendment and a fertilizer that meas- ure up to hygiene standards.

SUMMARY OF THE INVENTION

The method according to the invention for disinfecting sludge is characterized by what is pre- sented in claim 1.

The soil amendment according to the invention is characterized by what is presented in claim 14.

The fertilizer according to the invention is characterized by what is presented in claim 15. .

The invention is based on research work carried out in order to enhance the disinfection of sludge. In this connection, it was surprisingly discovered that the dosage of organic peroxide such as peroxyacid, hydrogen peroxide or their mixture with burnt lime is extremely well applicable to make sludge hygienic and bind ammonia released in con-neOtion with the purification of sludge produced in the purification of wastewater.

Sludge in this application means the purification plant sludge from wastewater facilities, such as the excessive sludge from activated sludge facilities and the outlet sludge such as digestate from bio- gas facilities as well as the sludge produced in industrial processes such as in the paper industry and the food industry.

In one embodiment of the invention, the dry weight of the sludge ranges approximately from 20 to 40 w-%, typically approximately from 25 to 35 w-%. The sludge is mechanically dewatered for example by cen- trifugation, by a screw, filter belt or chamber press or by other mechanical dewatering apparatus applicable for the dewatering of sludge. The sludge is preferably dewatered by centrifugation .

By the method according to the invention, pathogenic microbes, such as E.coli and salmonella, are disinfected out of the sludge in sucb a way that the treated sludge measures up to official standards. By the method, part of the ammonia released in the treatment is also bound into water soluble nitrogen compounds remaining in the treated sludge and thus im- proving the nutritional properties of the sludge.

In the method according to the invention, burnt lime CaO is added to the sludge and the sludge is agitated until the pH of the sludge mixture exceeds 11.

The burnt lime CaO readily reacts With water in the sludge, producing calcium hydroxide:

■ CaO + H ₂ 0 - Ca (OH) ₂

This raises the pH of the sludge and the hygienic- making effect of the method is partly based on this raising of the pH value, as infectious agents are not able to survive or multiply at the high (>10) pH lev- els. The reaction is exothermic, which raises the temperature of the sludge mixture further. The rise of temperature enhances making the sludge hygienic. The addition of burnt lime releases ammonia from the sludge as the pH and the temperature of the sludge rise .

Then, peroxide is added to the sludge mixture to bind the ammonia formed in the agitation step. The peroxide used in the method according to the invention may be any compound containing a peroxide group [0-0] .

In one embodiment of the invention, peroxyacid is added to the sludge mixture to bind the ammonia formed in the agitation step. The chemical formula of peroxyacid is RC0 ₃ H, where R is an organic radical such as a saturated or unsaturated, straight-chain, branched or cyclic hydrocarbon. Peroxyacid binds · ammonia by forming water soluble ammonium salts with it. The water-soluble ammonium salts of peroxyacids are well applicable as a nitrogen source for plants. Peroxyacids furthermore constitute antibacterial agents , The most typical peroxyacids include peracetic acid and peroxyformic acid.

In one embodiment of the invention, peracetic acid is added to the sludge mixture to bind ammonia formed in the agitation step.

In one embodiment of the invention, hydrogen peroxide is added to the sludge mixture.

In one embodiment of the invention, a mixture of peroxyacid such as peracetic acid and hydrogen peroxide is added to the sludge mixture to bind ammonia formed in the agitation step.

Peracetic acid binds ammonia by forming water soluble ammonium acetate salts with it. In addition, peracetic acid oxidates bacteria coli and also many other bacteria and unwanted microorganisms, such as the bacteria Salmonella and Legionella and Giardia parasites.

Peroxyacid such as peracetic acid or hydrogen peroxide also reduces the possible odor annoyances of the treated sludge.

Burnt lime is added to the sludge by efficient agitation. Burnt lime is added as a powder. For agitation, any known agitator for use in an industrial agitation process may be used.

Peroxide such as peroxyacid, hydrogen peroxide or their mixture is added to the sludge mixture as a solution. In the method according to the invention, any strength of peroxyacid or hydrogen peroxide may be used.

In one embodiment of the invention, peracetic acid of 2-15 w-%, preferably commercial peracetic acid of 12 w-%, is used.

The addition of peroxide, such as peroxyacid, hydrogen peroxide or their mixture, is carried out in such a way as to make mixing as immediate and efficient as possible. The addition can be made for example by spraying or injecting and over the entire surface of the sludge mixture. Dosage can be enhanced by agitation .

In one embodiment of the invention, the agitation of CaO and the sludge is continued until the pH exceeds 12.

At the beginning of the method, the temperature of the sludge being treated typically ranges from 20 to 37 °C depending on the origin of the sludge. The temperature of the outlet sludge, digestate, from a biogas facility is approximately 37°C and the temperature of the activated sludge from a water purification plant ranges approximately from 20 to 25 °C. In one embodiment of the invention, the agitation of the sludge mixture is continued until the temperature ris- es for 10 - 30 °C, preferably for 15 - 20 °C and most preferably for approximately 15 °C.

In one embodiment of the invention, the sludge is agitated before the addition of burnt lime CaO.

In one embodiment of the invention, the sludge is agitated in connection with the addition of peroxide such as peroxyacid, hydrogen peroxide or their mixture and/or after the addition.

Burnt lime CaO is added to the sludge for approximately 5 - 20 w-% of the dry matter of the sludge, preferably for approximately 7 - 13 w-% and most preferably for approximately 10 w-% of the dry matter. The amount of burnt lime may also be greater or smaller depending on the properties of the sludge being treated.

Peroxide such as peroxyacid, for example peracetic acid, hydrogen peroxide or their mixture is added to the sludge for approximately 0.1 - 0.3 w-% of the dry matter of the sludge, preferably for approximately 0.15 - 0.2 w-% and most preferably for approximately 0.18 w-% of the dry matter. The amount of peroxide may also be greater or smaller depending on the properties of the sludge being treated.

In the method according to the invention, pH is kept at least at the value of 11 until the amount of infectious microorganisms becomes smaller than the maximum levels of official regulations. In one embodiment of the method of the invention, pH is kept at least at the value of 11 for approximately from 1 to 2 hours. In another embodiment of the invention, ^' pH is kept at least at the value of 12 for approximately from 1 to 2 hours . : : ^" .

In one embodiment of the method, the sludge/ such as dewatered purification plant sludge from a water supply facility, is led from a silo to a mixing device to which burnt lime and peroxide such as peroxyacid,, hydrogen peroxide or their mixture are led from their feeding tanks. In the mixing tank, pH and temperature are measured by pH and temperature measure ^¬ ment sensors and the sludge purification method is adjusted on the basis of the measurement results.

In the method according to the invention, the addition and mixing of burnt lime and peroxide such as peroxyacid, hydrogen peroxide or their mixture is carried out either as a continuous process or as a batch process in the mixing tank. The mixing efficiency in the mixing tank is sufficient to guarantee the smooth mixing of burnt lime, peroxide and the sludge in order to ensure the desired end result.

In the method according to the invention, the amounts of dosage and the feeding order of burnt lime and peroxide such as peroxyacid, hydrogen peroxide or their mixture can be varied depending on the process of the facility and the quality of the sludge being treated.

In one embodiment of the invention, burnt lime and peroxide such as peroxyacid, hydrogen perox ^¬ ide or their mixture are mixed to the sludge in the mixing device in a stepwise manner.

In one embodiment of the invention, peracetic acid is added to the sludge and burnt lime powder is fed to the sludge treated with peracetic acid.

The method according to the invention is mi- crobiologically efficient. The method according to the invention substantially speeds up and enhances the treatment of sludge at the facilities. Immediate transportation of the sludge out of the area of the facility saves logistics costs because intermediate storing is not necessary. The sludge purified by the method does ^' not contain harmful microbes and is -of economic value as a soil amendment and a fertilizer due to, among others, the soluble nitrogen it contains. Equipment costs are low and the monitoring of the hygienization process is easy.

The system according to the method according to the invention is simple to install, an inexpensive and light solution readily moffiitabJe to a water purification facility either as a continuously operated part .

Further, the method according to the inven- tion simultaneously raises the basicity of the sludge, binds part of the nitrogen in the untreated sludge to the treated sludge and removes harmful organic compounds from the sludge.

The soil amendment according to the invention is produced by the sludge purification method according to the invention. The soil amendment is produced by a method where burnt lime CaO is added to sludge, the sludge mixture is agitated until the pH of the mixture exceeds 11, preferably exceeds 12; and

peroxide is added to the mixture to bind ammonia formed in the agitation step.

As peroxide, peroxyacid such as peracetic acid, hydrogen peroxide or a mixture of a peroxyacid and hydrogen peroxide can be used.

In one embodiment of the invention, the agitation of the sludge mixture is continued until the temperature rises for 10 - 30 °C, preferably for 15 - 20 °C and most preferably for approximately 15 °C. ^:

The fertilizer according to the invention is- produced by the sludge purification method according to the invention. The fertilizer is produced by- a method where burnt lime CaO is added to sludge,

the sludge mixture is agitated until the pH of the mixture exceeds 11, preferably exceeds 12;. and

peroxide is added to the mixture to -bind ammonia formed in the agitation step. As peroxide, peroxyacid such as peracetic acid,, hydrogen peroxide or a mixture of a peroxyacid and hydrogen peroxide can be used.

In one embodiment of the invention, the agi- tation of the sludge mixture is continued until the temperature rises for 10 - 30 °C, preferably for 15 - 20 °C and most preferably for approximately 15 °C.

The method produces disinfected sludge with microbe levels falling below the highest allowable levels of official regulations.

The dry matter content of the sludge rises approximately for 10% in the treatment, which improves the. processability of the sludge as a soil amendment and a fertilizer. After disinfection, the sludge can be directly taken out to be spread to the field or it may be clamped on the field during a period when sludge may not be spread to the field.

Further, the method according to the invention provides a soil amendment and a fertilizer having an improved water retention capacity and increasing humus. The soil amendment and the fertilizer according to the invention raise the pH of fields, lime the fields and act as a nitrogen source. - LIST OF FIGURES

Fig. 1 represents a flowchart of the method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Example 1

The method according to one embodiment of the invention is illustrated in Fig. 1 showing a mixing tank (1), a burnt lime dosage tank (2) and a peroxyacid dosage tank (3) from which burnt lime and peracetic acid are dosed into the mixing tank by a suitable dos- age means such as a pump. A pH sensor and a temperature sensor are installed in the mixing tank. The feeding of sludge into the mixing tank is arranged from a silo (5) . The purified sludge in led. out of the mixing tank through an outlet sludge conduit (4) . The mixing tank (1) which is provided with an effective agitator applicable for agitating the sludge may be movable or stationary.

In example 1, the efficiency of purification of the method according to the invention is tested.

The sludge being used is activated sludge produced in the purification of wastewater. The initial temperature of the sludge is approximately 25°C and the amount is approximately 10 tons. The purifica- tion plant sludge may contain approximately 100000 - 4000000 cfu/kg of a pathogenic microbe, e.g. E.coli.

The purification plant sludge is added to the mixing tank (1) from the silo (5) . The purification plant sludge is centrifuged to a dry matter content of approximately 30w-% before being added to the mixing tank. Burnt lime is evenly added as a powder to the mixing tank from its dosing tank (2) while agitation is carried out the whole time. 300kg of burnt lime are added. In the method, the pH of the sludge mixture is measured by any pH meter applicable for the measurement of pH. In the method, the temperature of the sludge mixture is also measured by any sensor applicable for the measurement of temperature. The pH and temperature measurements may be carried out continu- ously or at desired intervals. According to the embodiment of the example, the measurements were carried out at one minute intervals.

The sludge mixture is agitated .until the pH of the mixture exceeds 12 and approximately for one hour. The temperature of the sludge mixture rose approximately for 15°C during the agitation. Then, 200kg of peracetic acid solution of approximately 3w-% are evenly injected from the dosing tank (3) to the mixing tank onto the surface of the sludge mixture being treated using separate nozzles.

In an alternative method, the peracetic acid solution is in the mixing tank .

Then, the sludge mixture is agitated further approximately for one hour. The amounts of the solution and the powder are monitored on a continuous ba- sis by means of a scale provided in the mixing tank.

In the method, the sludge is agitated for approximately 1 - 2 hours in total when the sludge being treated is approximately 10 tons. Then, ^' the treated sludge is led through the conduit (4) directly into a delivery truck for transportation.

Infectious agents and other microorganisms as well as nitrogen, phosphorus and heavy metal levels for each determined component were analyzed in the sludge purified according to Example 1 using an ac- credited method. The results are presented in Table 1.

The table shows that neither micro-organism nor heavy metal levels exceed the maximum level limits imposed on soil amendments and fertilizers.

The dry matter content of the sludge rose for 10% during treatment.

The results clearly show that the _. quality of the disinfected sludge is excellent and that the method according to the invention is efficient.

Example 2

A soil amendment was produced by treating purification plant sludge from a wastewater facility according to Example 1. The nutrient levels of the soil amendment were analyzed and the results are shown in Table 2.

The heavy metal levels of the product do not exceed the maximum level limits imposed on soil amendments. The product is applicable as a soil amendment in field cultivation, development of green areas and landscaping.

Example 3

In example 3, the efficiency of purification of the method according to the invention is also tested.

The sludge being used is activated sludge produced in the purification of wastewater. The initial temperature of the sludge is approximately 25°C and the amount is approximately 10 tons.

Purification is carried out as in Example 1. The purification plant sludge is centrifuged to a dry matter content of approximately 30w-% before being added to the mixing tank. 300kg of burnt lime are add- ed.

The sludge mixture is agitated until the pH of the mixture exceeds 12 and approximately for one hour. The temperature of the sludge mixture rose approximately for 15°C during the agitation. Then, 15kg of hydrogen peroxide solution of approximately ^: 36w-% are evenly injected from the dosing tank ^" (3) to the mixing tank onto the surface of the sludge mixture being treated using separate nozzles. ^: Then, the sludge mixture is agitated further approximately for one hour. The amounts of the solution and the powder are monitored on a continuous basis by means of a scale provided in the mixing tank.

In the method, the sludge is agitated for approximately 1 - 2 hours in total when the sludge being treated is approximately 10 tons. Then, the treated sludge is led through the conduit (4) directly into a delivery truck for transportation and use as a soil amendment.

The nutrient levels of the sludge purified according to Example 3 were analyzed and the results are shown in Table 3.

The invention is not limited merely to the exemplary embodiments referred to above; instead, many variations are possible within the scope of the in- ventive idea defined by the claims.