The invention relates to a method and arrangement for treatment of biomass, such as sludge, according to the preambles of the enclosed independent claims.

Biomass, such as sludge, which is obtained, for example, as a by-product in treatment of waste water or from fish hatcheries, contains valuable nutrients, which make the sludge potentially interesting fertilizer in agriculture. However, the sludge may also contain pathogenic micro-organisms and parasites, such as fecal coliforms, Salmonella spp., helminth ova. The legislation requires that if the sludge is intended to be used as a fertilizer, it must be treated or hygienised so that there is no risk of pathogens or parasites for humans or environment.

It is known to treat sludge with peracetic acid. However, peracetic acid is difficult to transport long distances, as it is relatively corrosive. Thus there exists a need for a simpler, potentially safer, method for treatment of sludge, which would still provide excellent treatment result, i.e. effectively reduce the number of pathogenic microorganisms and parasites in the sludge. For example, J. A. Barrios et al., Journal of Residuals Science & Technology, Vol. 1 , No. 1 , 2004 discloses treatment of sludge with peracetic acid to reduce the microbial content.

An object of this invention is to minimise or even eliminate the disadvantages existing in the prior art.

An object of the invention is also to provide a method and arrangement which provide simple and effective treatment of biomass, especially sludge, such as digested or non-digested sludge.

These objects are attained with an invention having the characteristics presented below in the characterising parts of the independent claims. Typical method according to the present invention for treatment of biomass comprises

- obtaining biomass having a dry solids content of at least 3 weight-%,

- mixing performic acid to the biomass, and

- allowing performic acid to react with the biomass.

Typical use of performic acid according to the present invention is for treatment of biomass, preferably sludge, such as digested or non-digested sludge, having dry solids content of 3 - 50 weight-%.

Typical arrangement according to the present invention for treatment of biomass comprises

- a mixing apparatus, such as a mixing tank with a mixer, a screw mixer or a horizontal mixer,

- means for feeding biomass having dry solids content of 3 - 50 weight-% to the mixing apparatus.

- a preparation unit for performic acid, and

- means for feeding performic acid prepared in the preparation unit to the mixing apparatus.

Now it has been surprisingly found out that adding and mixing of performic acid to the biomass effectively reduces the amount of pathogenic micro-organisms. In this manner treated biomass is effectively obtained. The use of performic acid is easy, as it can be prepared in situ by commercially available preparation apparatuses. Thus there is no need for long distance transports of treatment chemicals from a chemical manufacturer to the biomass treatment station, which effectively reduces risk for accidents and/or handling mistakes during transport.

In context of the present application the term "biomass" comprises and is selected from sludge, such as digested sludge or non-digested sludge; sludge from a fish hatchery or from a fish farm; digested biowaste; faeces contaminated substances, such as septic tank waste or manure; industrial waste comprising pathogenic micro-organisms such as E. coli and/or Salmonella spp. ; or biosolids. Especially, biomass may be sludge, which is digested sludge, non-digested sludge or dewatered sludge, non-digested sludge being preferred, non-digested dewatered sludge being more preferred. According to one embodiment, biomass is preferably sludge from municipal waste water treatment, still more preferably non-digested sludge from municipal waste water treatment.

According to another preferable embodiment of the invention the biomass is sludge from a fish hatchery or from a fish farm. These sludges are rich in nitrogen and/or phosphorous and may provide good fertilizers for agricultural purposes after treatment with performic acid. Treatment with performic acid reduces the risk of spreading of harmful microbes and/or parasites, and guarantees safe use of fish hatchery sludge as fertilizer.

The biomass, such as sludge, which is treated, may have a dry solids content of at least 3 weight-%, preferably at least 5 weight-%, more preferably at least >10 weight-% (before treatment with performic acid). The biomass may be dewatered before the addition of performic acid for obtaining optimal dry solids content. According to one embodiment of the invention the biomass, preferably digested or non-digested sludge from waste water treatment, is dewatered to a dry solids content of 10 - 50 weight-%, preferably 15 - 30 weight-%, before adding the performic acid to the biomass. Use of relatively dry dewatered biomass, such as sludge, is preferred as it makes the treatment of biomass more economical. For example, it is possible to treat sludge at the waste water treatment plant or at a separate sludge treatment station, which is situated at different location than the waste water treatment plant. For example, the dewatered sludge can be transported from the treatment plant to a separate sludge treatment station.

According to one embodiment of the invention the dry solids content of the treated biomass (after treatment with performic acid) is at least 2.5 weight-%, typically at least 4 weight-%, more typically at least 8 weight-%. The dry solids content of the treated biomass is preferably in the range of 5 - 45 weight-%, more preferably 10 - 25 weight-%. According to one embodiment of the invention the biomass to be treated is dewatered mechanically by using a centrifuge, a belt filter press or a screw press. Mechanical dewatering provides effective results without increasing the chemical load of the process. Furthermore, most waste water treatment plants have suitable mechanical dewatering apparatuses, and no new investments are needed.

Performic acid, CH ₂ 0 ₃ , is used as an aqueous solution. Typically the performic acid has a concentration at least 10 %, calculated as weight to volume, typically around 13.5 %, calculated as weight to volume. Preferably the aqueous performic acid solution is used as an equilibrium solution.

According to one embodiment of the invention performic acid is used in amount of 10 - 50 kg/ton biomass dry solids, preferably 15 - 45 kg/ton biomass dry solids, more preferably 20 - 40 kg/ton biomass dry solids. The dose is given as 100% performic acid. In case small dose of performic acid is used, it may be necessary to prolong the reaction time for obtaining the desired reduction in amount of pathogenic micro-organisms. In this application the term "reaction time" covers not only the actual mixing of performic acid and the biomass, but also the following residence time during storage, e.g. in a storage tank, silo, container, reservoir or pile, while the number of pathogenic micro-organisms is still decreasing. According to one embodiment of the invention performic acid is allowed to react with the biomass, such as sludge, at least 25 min, preferably at least 30 min, more preferably at least 60 min, min and/or until the level of E. coli is < 1000 cfu/g, preferably < 500 cfu/g, more preferably <250 cfu/g. In case the dosage of performic acid is < 15 kg/ton biomass dry solids, the reaction time is at least 6 days. It has been observed that the amount of pathogenic micro-organisms is actually reducing during the days following the addition and mixing of the performic acid to the biomass. According to one embodiment it is possible to dose additional chemicals, such as acids or other oxidants, to the biomass before adding the performic acid to the biomass. For example, the dosage of the performic acid may be optimised by lowering the pH of the biomass before the addition of performic acid. pH of the biomass may be lowered by appropriate addition of another acid before the addition of performic acid.

The arrangement for treating biomass comprises a mixing apparatus, which may be a mixing tank with a mixer, a horizontal mixer or a screw mixer. The mixing tank is typically equipped with a blade mixer. The type of mixing apparatus depends on the dry solids content of the biomass to be treated. In case the biomass to be treated is relatively dry, i.e. dewatered sludge, the mixing apparatus is typically a screw mixer or a horizontal mixer.

The preparation unit for performic acid is commercially available from Kemira Oyj, sold under name DesinFix technology. Performic acid is thus prepared in the immediate vicinity of the mixing apparatus, which guarantees the high chemical efficiency of the performic acid. Performic acid is led to the mixing apparatus through suitable connections and fed to the mixing apparatus through suitable feeding means. During mixing the pH of the biomass decreases, typically to a pH value < 6, more typically to a value of 3 - 6. At the same time the temperature of the mixture is increasing, which also promotes the reduction of pathogenic microorganisms.

The method according to the invention may be performed as a batch process or as a continuous process.

According to one embodiment of the invention the arrangement further comprises at least one storage tank, container, silo, reservoir or pile or the like for storing the treated biomass, such as sludge. After the mixing of the biomass and performic acid, the treated biomass may be transferred to the storage tank, silo or to the storage pile, where it may be stored before its use as a fertiliser. At the same time the desired reduction in amount of pathogenic micro-organisms may continue in the storage tank or container, vessel, silo, reservoir or pile or the like. It is also possible to transfer the treated biomass directly to the transportation vehicle, with which it is transported to the final destination of use, e.g. field or farmland. The arrangement may also comprise one or several receiver tanks, containers, silos, reservoirs, piles or the like, which are arranged prior to the mixing apparatus. Especially when the separate biomass treatment station is situated at a different location than the origin of generation of the biomass, e.g. municipal water treatment, the receiver tank(s), silos, pile(s) or the like may function as buffer stock for the biomass to be treated.

It is possible that the treated biomass is stabilised with lime, CaO. According to one preferred embodiment, non-digested sludge from municipal waste water treatment is dewatered mechanically by using a centrifuge, a belt filter press or a screw press. After the dewatering, the dewatered sludge having dry solids content > 10 weight-% may be treated directly by mixing with performic acid, or the dewatered sludge may be stored in a container, silo or pile before treatment with performic acid. The treatment with performic acid may be performed in connection with the municipal waste water treatment or at a separate sludge treatment station. The dewatered sludge may be transported to the separate sludge treatment station by using a transport vehicle, such as truck, lorry or the like. After the mixing with performic acid and appropriate reaction time, under which the level of E. coli in the treated sludge is reduced to < 1000 cfu/g, preferably < 500 cfu/g, the treated sludge is either stored for further use, or transported directly for final destination of use.

According to one embodiment of the invention performic acid is used for treatment of biomass, preferably sludge, such as digested or non-digested sludge, having dry solids content of 3 - 50 weight-% for reducing the level of E. coli in the treated biomass to < 1000 cfu/g, preferably < 500 cfu/g, more preferably <250 cfu/g, still more preferably < 200 cfu/g, sometimes even < 100 cfu/g. According to one embodiment of the invention the method does not comprise a neutralisation step of the treated biomass, e.g. sludge. This means that the treated biomass is not treated with a base after the reaction with performic acid. EXPERIMENTAL Example 1

1000 g of clarification sludge, which was not anaerobic fermented and which had dry solids content of 4 weight-%, was poured into a 2 liter beaker. Blade mixer was used to mixing the sludge with mixing speed 300 rpm. Performic acid equilibrium solution having active performic acid, PFA, concentration 13.5 weight-to-volume-% (Kemira DEX-135) was dosed to the sludge with pipette. The dosing amount was 10, 20 and 30 kg active performic acid/ton dry solids. Mixing time was 30 minutes.

Samples were taken to the microbial analysis. E. coli was analysed with Chromocult® Coliform agar, with E. co///Coliform Supplement to avoid the overgrowth of gram-positive bacteria, Pseudomonas or Aeromonas spp.. Salmonella was analysed according to the method NMKL 71 :1999 Salmonella. First microbial analysis was done on the next day from the treatment. Meanwhile the sample was stored in cold room at 5°C. The same analyses were also done after 7 days from the treatment. Meanwhile the samples were stored in room temperature, caps open. pH was also determined always when microbial analyses were done.

Results are given in Table 1 . Example 2

2000 g of dewatered sludge, which was not anaerobic fermented and which had dry solids content of 24 weight-% was put into a Lodige mixer. Performic acid equilibrium solution having active performic acid, PFA, concentration 13.5 weight- to-volume-% (Kemira DEX-135) was dosed to the mixer with syringe. The dosing amount was 10, 20 and 30 kg active performic acid/ton dry solids. Mixing time was 60 minutes.

Samples were taken to the microbial analysis. E. coli was analysed with Chromocult® Coliform agar, with E. co///Coliform Supplement to avoid the overgrowth of gram-positive bacteria, Pseudomonas or Aeromonas spp.. Salmonella was analysed according to the method NMKL 71 :1999 Salmonella. First microbial analysis was done on the next day from the treatment. Meanwhile the sample was stored in cold room at 5°C. The same analyses were also done after 7 days from the treatment. Meanwhile the samples were stored in room temperature, caps open. pH was also determined always when microbial analyses were done, sample being diluted 1 :1 with water before pH measurement.

Results are given in Table 1 . Table 1 . Results of Example 1 and Example 2.

Results given in Table 1 shown clear reduction in amount of E. coli and Salmonella. Desired reduction may be achieved even with lower dosage of performic acid, provided that the reaction time is long enough.

Even if the invention was described with reference to what at present seems to be the most practical and preferred embodiments, it is appreciated that the invention shall not be limited to the embodiments described above, but the invention is intended to cover also different modifications and equivalent technical solutions within the scope of the enclosed claims.