WO2023161275A1

WO2023161275A1 - Processing sludge from paper pulp manufacturing plants using beetle larvae

Info

Publication number: WO2023161275A1
Application number: PCT/EP2023/054421
Authority: WO
Inventors: Nils ÖSTERSTRÖM
Original assignee: Tebrito Ab
Priority date: 2022-02-22
Filing date: 2023-02-22
Publication date: 2023-08-31

Abstract

There is provided a method for processing biosludge from a paper pulp manufacturing plant comprising the steps of providing biosludge from the paper pulp manufacturing plant, and feeding the biosludge to a culture of larvae from the order Coleoptera (beetles).

Description

PROCESSING SLUDGE FROM PAPER PULP MANUFACTURING PLANTS USING BEETLE

LARVAE

Field of the invention

This invention relates to the processing of sludge form paper pulp manufacturing plants using larvae from insects, in particular larvae from beetles.

Background

Pulp- and paper manufacturing plants produce about 6 million tons (dry substance) of pulp and paper biosludge (PPBS) every year.

PPBS is frequently contaminated with heavy metals such as lead, mercury and cadmium, and other potentially harmful substances.

PBBS is frequently handled by incinerating it in an incinerator in the pulp or paper manufacturing plant. Many pulping plants has an incinerator for this purpose. However, incineration costs energy and money and also produces smoke. Nitrous gases such as nitrogen monoxide is produced at incineration.

PPBS is sometimes composted. However, this also is an environmental problem because of leakage of harmful substances. Moreover, biosludge tends to smell which creates yet another environmental problem.

Hence, PPBS is a serious waste problem. Compared to other types of biosludge, for example biosludge from household wastewater, PPBS has elevated levels of cellulose, lignin, sulfur and zinc.

PPBS has been tried as feed for larvae of Black Solider Fly (Hermetic/ illucens), with discouraging results. The study concluded that the nutrients in PPBS was not readily available to the larvae. (Norgren, et al, Bio-sludge from the Pulp and Paper Industry as Feed for Black Soldier Fly Larvae: A Study of Critical Factors for Growth and Survival. Waste Biomass Valor 11, 5679-5685 (2020)).

There is a need for improved methods for handling PPBS and in particular separation of heavy metals from PPBS.

Summary of invention

In a first aspect of the invention there is provided a method for processing biosludge from a paper pulp manufacturing plant comprising the steps of a) providing biosludge from the paper pulp manufacturing plant, b) feeding the biosludge to a culture of larvae from the order Coleoptera (beetles). The inventors have surprisingly found that biosludge form paper pulp manufacturing plants can be used as feed for the Coleoptera larvae, in particular larvae of Tenebrio molitor (mealworm).

Using insect larvae to process pulp biosludge has several advantages. There is no need for incineration. This saves energy and limits release of nitrous gases. The problem of smell is reduced. There is no need for composting the biosludge. Moreover, heavy metals in the biosludge can be enriched and separated.

The larvae produced in the process can be used for example as fish feed, or for production of food for humans. The frass from the larvae can be used as compost. The frass is rich in chitin and can be used for production of chitosan, which is of high value. It has surprisingly been shown that the larvae eat the biosludge even though the biosludge may comprise heavy metals.

The biosludge may comprise lead at a concentration of at least 1 mg/kg dry substance, or cadmium at a concentration of at least 0.1 mg/kg dry substance, or mercury at a concentration of at least 0.01 mg/kg dry substance or copper at a concentration of at least 1 mg/kg dry substance or arsenic at a concentration of at least 0.25 mg/kg dry substance. The biosludge may comprise lead at a concentration of from 1 mg/kg dry substance to 100 mg/kg dry substance, or cadmium at a concentration of from 0.1 mg/kg dry substance to 15 mg/kg dry substance, or mercury at a concentration from 0.01 mg/kg dry substance to 0.5 mg/kg dry substance, or copper at a concentration of from 1 mg/kg dry substance to 100 mg/kg dry substance or arsenic at a concentration of from 0.25 mg/kg to 10 mg/kg dry substance.

The biosludge may comprise lead at a concentration of from 1 mg/kg dry substance to 50 mg/kg dry substance, or cadmium at a concentration of from 0.1 mg/kg dry substance to 10 mg/kg dry substance, or mercury at a concentration of from 0.01 mg/kg dry substance to 0.3 mg/kg dry substance, or copper at a concentration of from 1 mg/kg dry substance to 50 mg/kg dry substance or arsenic at a concentration of from 0.25 mg/kg to 5 mg/kg dry substance.

The biosludge comprises may comprise lead at a concentration of from 1 mg/kg dry substance to 10 mg/kg dry substance, or cadmium at a concentration of from 0.1 mg/kg dry substance to 5 mg/kg dry substance, or mercury at a concentration of from 0.01 mg/kg dry substance to 0.22 mg/kg dry substance, or copper at a concentration of from 1 mg/kg dry substance to 35 mg/kg dry substance or arsenic at a concentration of from 0.25 mg/kg to 1 mg/kg dry substance.

It has surprisingly been shown that the concentration of heavy metals is very low, even though the larvae are provided with biosludge that contains a heavy metal. The method may comprise the step of, at the end of a culturing period, separating frass and the larvae as separate fractions from the culture, where the concentration of a heavy metal in the larvae is at most: 0.001 mg lead/kg dry substance or 0.001 mg cadmium/kg dry substance or 0.001 mg mercury/kg dry substance or 0.01 mg copper/kg dry substance or 0.001 mg arsenic/kg dry substance.

The method may comprise the step of, at the end of a culturing period, separating frass and the larvae as separate fractions from the culture, and where the concentration of a heavy metal in the larvae is at most: 0.01 mg lead/kg dry substance or 0.015 mg cadmium/kg dry substance or 0.005 mg mercury/kg dry substance or 0.1 mg copper/kg dry substance or 0.02 mg arsenic/kg dry substance.

The method may comprise the step of, at the end of a culturing period, separating frass and the larvae as separate fractions from the culture, and where the concentration of a heavy metal in the larvae is at most: 0.015 mg cadmium/kg or 0.02 mg arsenic/kg dry substance.

The method may comprise the step of at the end of a culturing period separating frass and the larvae as separate fractions from the culture, and where the ratio, of the concentration of a heavy metal in the larvae to the concentration of the heavy metal in the frass, is at most 0.25, more preferably at most 0.1, for one of lead, cadmium, mercury copper or arsenic. Hence, the concentration of a heavy metal in the larvae is preferably at most a fourth of the concentration of the heavy metal in the frass. Hence, the concentration of a heavy metal in the larvae is preferably at most a fourth of the concentration of the heavy metal in the frass.

It has surprisingly been shown that the ratio of a heavy metal in the larvae in relation to the concentration of a heavy metal in the frass, is very low. The method may comprise the step of, at the end of a culturing period, separating frass and the larvae as separate fractions from the culture, and where the ratio, of the concentration of a heavy metal in the larvae to the concentration of the heavy metal in the frass, is from 0.001 to 0.25 for lead or from 0.002 to 0.25 for cadmium or from 0.01 to 0.25 to for mercury or from 0.001 to 0.25 for copper or from 0.005 to 0.25 for arsenic.

The method may comprises the step of, at the end of a culturing period, separating frass and the larvae as separate fractions from the culture, and where the ratio, of the concentration of a heavy metal in the larvae to the concentration of the heavy metal in the frass, is from 0.005 to 0.2 for lead or from 0.01 to 0.2 for cadmium or from 0.05 to 0.2 to for mercury or from 0.01 to 0.2 for copper or from 0.02 to 0.2 for arsenic.

The method may comprise the step of, at the end of a culturing period, separating frass and the larvae as separate fractions from the culture, and where the ratio, of the concentration of a heavy metal in the larvae to the concentration of the heavy metal in the frass, is from 0.007 to 0.2 for lead or from 0.015 to 0.2 for cadmium or from 0.1 to 0.2 to for mercury or from 0.01 to 0.2 for copper or from 0.04 to 0.2 for arsenic.

The larvae may in particular be from a species selected from Tenebrio molitor (mealworm beetle), Zophobas atratus (Giant mealworm), Alphitobius diaperinus (Lesser mealworm, buffalo worm), Pachnoda butana (Sun beetle), Pachnoda aemole (Sun beetle), Pachnoda marginata (Sun beetle), Zophobas morio (King worm, greater mealworm, Superworm, Mono), Sitophilus granarius (Wheat weevil), Sitophilus oryzae (Rice weevil), and Oryctes nasi- cornis (European rhinoceros beetle), where the larvae is from Tenebrio molitor (mealworm beetle) is preferred.

The biosludge may in particular be from a sulphate paper pulp manufacturing plant.

The larvae from Coleoptera species can surprisingly be used to break down biosludge that comprises a number of substances that are otherwise difficult to handle. The biosludge may comprise from 10 % to 40 % cellulose (dry substance). The biosludge may comprise from 10 % to 50 % lignin (dry substance). The biosludge may comprise zinc at from 50 mg/kg dry substance to 1000 mg /kg dry substance. The biosludge may comprises sulfur at from 0.1 to 2.4 % dry substance.

The larvae may be additionally fed a plant-based material, such as for example wheat bran. This increases the growth of the larvae.

The biosludge may be heat treated before feeding it to the larvae. This has the advantage of destroying bacteria that are undesirable in the culture.

In a second aspect of the invention there is provided a system for processing biosludge from a paper pulp manufacturing plant comprising a paper pulp manufacturing plant comprising a biological wastewater treatment plant and a wastewater outlet and biosludge separation means for separating biosludge from the wastewater outlet, the system further comprising an insect culturing facility comprising a culture of larvae from the order Coleoptera (beetles).

The system may comprise transportation means for transporting the biosludge from the biosludge separation means to the insect culturing facility. This facilitates transport from the biosludge separator to the insect culturing facility. The transportation means may be, for example, a conveyor belt, a screw or a pipeline.

The system may comprise a biosludge heater. This may be used to sterilize the bio sludge before providing it to the larvae.

Drawings

Fig. 1 is a schematic drawing of a system.

Fig. 2-3 are diagrams showing data from experiments. Detailed ion

Pulp biosludge may be collected from a paper pulp manufacturing plant, such as a kraft process pulping plant (sulphate process pulping plant), a sulfite process pulping plant or a thermo-mechanical pulping (TMP) process where a kraft/sulfate pulping process plant is preferred.

The paper pulp manufacturing plant may be a pulp plant that partially or completely uses fresh wood for the production of pulp to be used for paper production. Hence it may be a paper pulp manufacturing plant that does not use recycled paper as starting material.

The initial material for the pulp plant may be for example wood material from pine, spruce, birch or other trees, or any other species of tree that can be used for making paper pulp. Wood from a coniferous tree such as pine or spruce may be preferred, but wood from road-leaved trees may also be used. Suitable species include European Spruce (Picea abies) or Scots pine (Pinus sylvestris). Suitable species that are often used in north America include Pinus contorta and Abies balsamea. Other species of wood may be used in other parts of the world. The wood is typically debarked and then chipped to smaller fragments before entering the pulping process. Paper pulp is produced by fragmenting the chips to pulp using chemical mans (for example the sulfite process) or mechanical means (for example TMP). Large amounts of wastewater are produced during these steps.

In many, if not most, paper pulp plants the wastewater is treated using biological treatment before it is released. The wastewater may be collected from various steps of the pulping process and provided to a biological treatment plant.

During biological treatment the wastewater undergoes treatment by bacteria and other microorganisms as is known in the art of water treatment. For example, the wastewater may be kept in aerated or oxygenated tanks in order stimulate the growth of microbiota. Activated sludge treatment methods may be used. For example, a Moving Bed Biofilm Reactor (MBBR) may be used. Biosludge is produced as a byproduct from this process.

In some embodiments, the wastewater may also comprise wastewater from one or more paper production machines. The paper production machines use paper pulp to produce paper. Such wastewater from paper manufacturing may also have been processed in the biological treatment plant.

The biosludge may be separated from the wastewater from the paper pulp manufacturing plant.

The biosludge may be referred to as a complex mixture in that it may comprise numerous different compounds, since it is a byproduct of breaking down wood to paper pulp and bioprocessing. The composition of the pulp biosludge differs depending on the configuration of the paper pulping plant. The pulp biosludge may comprise water, lignin, cellulose, hemicellulose or carbohydrates, lipids, proteins, various other organic substances and nutrients, and heavy metals.

The biosludge is a mixture of dry substance and water. The water content may decrease during separation of the final biosludge used as feed, from the wastewater. Water content in the biosludge used as feed may be from 50% - 95 % more preferably from 60% - 80%. However, the biosludge may also be dried to provide a yet lower water content.

Dry substance content of the biosludge may be from 5 % to 50%, more preferably from 20% to 40%.

Dry substance content of the biosludge may be determined by weighing a sample of biosludge, heating the sample, for example to 105°C, to allow water to evaporate and weighting the dry substance that remains. The pulp biosludge may comprise cellulose. The cellulose content may be from 10 % - 40 % dry substance (DS), more preferably from 15 % to 35 % DS.

The pulp biosludge may comprise lignin. The lignin content may be from 10 % to 50 % DS more preferably from 15 % to 40 % DS.

Lignin, cellulose and hemicellulose may partly be in the form of fiber. The amount of fiber in the biosludge may be from 5% to 25%, dry substance. This percentage may comprise the weight of small amounts of other non-dissolved solids.

The pulp biosludge may comprise one or more heavy metals such as lead, mercury, zinc or cadmium. The concentration of lead may be from 1 mg/kg DS to 100 mg/kg DS, more preferably from 1 mg/kg DS to 20 mg/kg DS and most preferably from 3 mg/kg to 20 mg/kg. The concentration of cadmium may be from 0.1 to 15 mg/kg DS, more preferably from 3 mg/kg to 15 mg/kg, more preferably from 0.1 to 3 mg/kg DS. The concentration of mercury may be from 0.1 mg /kg DS to 1 mg/kg DS, more preferably from 0.01 to 0.5 mg/kg DS, more preferably from 0.01 to 0.1 mg/kg DS. The concentration of zinc may be from 50 mg/kg DS to 1000 mg/ kg DS or from 200 mg/kg to 1000 mg/kg dry substance. The concentration of copper may be from 1 mg/kg to 100 mg/kg, more preferably from 5 mg/kg to 50 mg/kg dry substance. The concentration of arsenic may be from 0.25 mg/kg to 10 mg/kg dry substance. In various embodiments, the pulp biosludge may have a maximum concentration of a heavy metal which is the various upper limits indicated above.

The sulfur content of the pulp biosludge may be from 0.1 to 2.5 % DS hence from 1000 mg/kg DS to 24000 mg/kg DS.

The pH of the pulp biosludge is typical around neutral pH (6.5-7.8) since it is obtained from biological wastewater. The larvae, in particular mealworm larvae may handle feed and environments that are quite acidic and slightly alkaline, so the pH of the pulp biosludge is likely not a problem. The biosludge may be supplemented with a fraction of fiber sludge. Fiber sludge is a sludge that is not from biological wastewater treatment but instead from wastewater comprising fiber from the pulping process which has not been processed using biological treatment.

Culturing of larvae from beetles, in particular Tenebrio molitor, is known as such.

The larvae are preferable larvae from a beetle (order Coleoptera). Preferably the larvae are from the genus Tenebrio, in particular from the species Tenebrio molitor (mealworm beetle). Other useful larvae are larvae from Coleoptera species include Zophobas atratus (Giant mealworm), Alphitobius diaperinus (Lesser mealworm, buffalo worm), Pachnoda butana (Sun beetle), Pachnoda aemole (Sun beetle), Pachnoda marginata (Sun beetle), Zophobas morio (King worm, greater mealworm, Superworm, Morio), Sitophilus granarius (Wheat weevil), Sitophilus oryzae (Rice weevil), and Oryctes nasicornis (European rhinoceros beetle).

Larvae may be cultured in containers. Larvae may for example be grown in standard mealworm growing plastic containers with dimensions of approx. 56 cm x 36 cm x 7 cm. However, larger (or smaller) containers may be used. The containers may be tray-like. The containers may be held in racks, such that a number of containers is held in each rack. The culturing bed is preferably not deeper than 10 cm, more preferably 5 cm.

The larvae are fed by distributing the pulp biosludge to the larvae. Preferably this is started at a predetermined time from hatching of the larvae which may be from 3 to 5 weeks from hatching.

The larvae eat the biosludge. The larvae may use the biosludge as a source of energy. The larvae may have an age of for example from four weeks of hatching to nine weeks of hatching, more preferably from five weeks of hatching to eight weeks of hatching. In various embodiments, the larvae may be provided with additional plant-based material as additional feed or, more preferably, as substrate for the larvae to live in and on. The plant-based material is preferably dry meaning that the water content of the dry material may be below 30%, more preferably below 10 %.

Non-limiting examples of useful plant-based material may be for example cereal grain or parts thereof, for example cereal flour, bran or cereal husks such as for example wheat bran, oat bran. Other suitable plant-based materials include rice, rice bran, rice flour, corn or other parts of the corn cob, oil seed husk, psyllium husk, bread or brewer's spent grain (BSG, draff). Wheat bran is a preferred plant-based material.

The plant-based material may be a material that comprises at least 30%, more preferably at least 50 % or at least 60 % of carbohydrates (DS). The plant-based material may have a protein content of at least 5 %, more preferably at least 10 % (DS). The plant-based material is preferably non-toxic. The plant-based material is preferably acceptable for use in culturing larvae. The plant-based material is preferably edible by the larvae. The plantbased material is preferably digestible by the larvae. The plant-based material may provide a three-dimensional structure for the larvae to crawl in.

The amount of plant-based material may be added at a ratio to the amount of biosludge. The weight of plant-based material may be from 30%, more preferably from 50% and most preferably from 80% of the weight (including water) of the biosludge provided to the larvae. The weight of the plant-based material may, in some embodiments, be at most 200% more preferably at most 100% and most preferably at most 50% of the weight of the biosludge.

The larvae may be given a total weight amount (wet weight) of biosludge per 1 kg produced final meal worm weight (fresh live larvae). The total weight amount of feed is given over a number of weeks, until harvest of the larvae, for example during weeks 3-8 from hatching of the larvae for Tenebrio molitor. The total weight amount may be, up to 3kg, 2,5 kg or 2 kg such as from 0.5 kg to 3 kg, or from 0.5 to 2.5 kg or from 0.5 kg to 2.0. kg. This amount is preferably provided during the culturing period. This corresponds to about from 100 g to 400 g, or from 100 g to 300 g or from 100 g to 200 g dry substance of biosludge.

Addition of water is typically not necessary as the pulp biosludge usually comprises sufficient water. However, if necessary, additional water may be provided to the larvae.

The larvae grow during culturing. The larvae may be harvested at around nine weeks from hatching, in particular for Tenebrio molitor. The culturing period may be at least 7, more preferably 8 weeks and most preferably at least 9 weeks from hatching. The culturing period may be ended when the larvae has reached a predetermined average weight, which may be chosen freely, but which may be for example from 60 to 140 mg, in particular for Tenebrio molitor. The culturing period may be limited to at most 10 or 11 weeks. The larvae may be fed the biosludge continuously during the culturing period but in some embodiments it may be preferably to provide the biosludge starting from weeks 2 or 3 from hatching.

Larvae can be harvested using methods known in the art. The method may comprise the step of at the end of a culturing period separating the frass and larvae as separate fractions from the culture. The frass can be separated from the larvae using suitable methods, for example a sieve. Machinery can be used to separate out dead larvae as is known in the art. Mealworm sorting machines that separate living larvae from frass and residues are known in the art.

At harvest, at least three fractions may be obtained: living larvae, frass, and residues where residues may comprise dead or immobile larvae or parts of these, remaining feed, exoskeletons. "Living larvae" may be defined as larvae that are able to move themselves and attach themselves to a surface. The frass fraction may be further stored and /or processed, in particular when heavy metals have been enriched in the frass fraction. Surprisingly, heavy metals are enriched in the frass fraction in relation to the living larvae fraction. The concentration of a heavy meatal in the living larvae fraction may be below a certain limit as described herein. Moreover, the ratio of concentration of a heavy metal in the living larvae fraction to the concentration of a heavy metal in the frass fraction may, in some embodiments be at most 0.5, more preferably 0.25 and even more preferably 0.1. The enriched heavy metal is preferably a heavy metal that is present in the biosludge.

The method may comprise the step of determining the concentration of a component of the biosludge, the larvae or the frass, in particular the concentration of a heavy metal.

The larvae, in particular mealworms, may be used for example as fish feed. Frass can be used for improving soil. The chitin content of the frass is useful for this purpose. Larvae may also be used for human consumption.

The weight of the frass is higher than the weight of larvae at harvest. The weight of frass is typically from 2 to 4 times, in particular the weight of larvae. The frass may make up from 45 % to 70 % of the total weight of the harvested material, in particular from 50 % to 65 %.

The concentration of various components of the biosludge and the plant-based material, may be determined as is known by a person skilled in the art.

Heavy metal content is preferably determined with the use of ICP-SFMS according to SS EN ISO 17294-1, 2 (mod) and United States EPA method 200.8. Dissolution was carried out in a microwave oven in closed Teflon containers using HNO3 /H2O2/HF.

Lignin content may be determined using the Klason method. A solid biomass sample is hydrolyzed, and the acid-insoluble residue is filtered out to determine the amount of Klason lignin. Acid-soluble lignin is determined from the hydrolysate using UV-VIS- spectrophotometer at 205 nm. Cellulose content is determined based on insolubility of cellulose in water and its resistance to action of dilute acids and bases. The sample is degraded with a mixture of nitric acid and acetic acid and boiled in apparatus that contained a condenser. The solution is then filtered through a funnel. Then the filter paper containing an insoluble residue is dried in oven and weighed.

System 100 comprises a paper pulp manufacturing plant 1. Paper pulp manufacturing plant 1 is typically industry-scale and may comprise several separate buildings or other structures. The paper pulp manufacturing plant 1 comprises various subsystems for producing paper pulp, for example a chipper, a boiler, etc. These subsystems produce wastewater. At least some wastewater is collected and treated in biological treatment plant 2. During biological treatment the wastewater undergoes treatment by bacteria and other microorganisms as is known in the art of water treatment. For example, the wastewater may be kept in aerated or oxygenated tanks in order stimulate the growth of microbiota. Activated sludge treatment methods may be used. For example, a Moving Bed Biofilm Reactor (MBBR) may be used. Pulp biosludge is produced as a byproduct from this process.

The pulp biosludge is separated from the wastewater using sludge separation means 3. Sludge separation means 3 may be any means that separates sludge from wastewater produced in the biological treatment plant 2. Hence pulp biosludge separation reduces the water content. Sludge separation means 3 may use any suitable technology such as for example sedimentation, centrifugation or pressing.

The pulp biosludge is then transported to the insect culturing facility 4 using transportation means 5. Sludge transportation means 5 may comprise for example a conveyor belt, a screw or a pipeline, where a pipeline is more suitable for sludge with high water content. A pipeline may have a pump. Insect culturing facility 4 comprises the facilities for culturing the larvae, in particular 7e- nebrio molitor based on pulp biosludge. The insect culturing facility 4 comprises a culture of larvae. The culture of larvae preferably comprises a large number of larvae. The insect culturing facility 4 may be able to produce thousands of tons of larvae per year.

The insect culturing facility 4 may be housed in a separate building from the paper pulp manufacturing plant 1. The insect culturing facility 4 may comprise equipment for providing a controlled climate for the larvae, with regard to, for example, temperature, humidity and lighting.

The insect culturing facility 4 is preferably arranged to separately culture larvae in different stages of development simultaneously. This way here will always be larvae that are in a stage of the life cycle to receive the pulp biosludge.

The environment in the insect culturing facility 4 is preferably controlled with regard to temperature, humidity and carbon dioxide and light /dark cycle as is known in the art of larvae culturing, in particular the art of culturing of Tenebrio molitor (mealworm). For example, a suitable temperature may be from 20°C to 35° C, more preferably from 23° C to 32° C. The humidity may be kept at for example from 55% to 70 % relative humidity.

Preferably system 100 comprises a biosludge heater 6 for heating the biosludge in order to kill off microorganisms, such as for example Legionella spp and Salmonella spp. Preferably the biosludge is heated to at least 70° C, more preferably at least 72°C. The biosludge may be heated in a batch process in an oven or a tank, for example. A tank may be stirred while heating to make sure that the sludge is heated evenly. The biosludge is heated before it is fed to the larvae, but should be allowed to cool down before it is provide to the insects. Samples may be taken and analyzed using microbiological methods in order to make sure that relevant bacteria have been eliminated. Upper and lower limits of the various intervals herein can be freely combined within the limits set out in the claims. The arithmetic precision of the numerical values can be increased by one or two digits for all values given in the present application. Hence, a value of given as e.g. 0.1 % can also be expressed as 0.10 or 0.100 %.

It is realized that everything which has been described in connection to one embodiment is fully applicable to other embodiments, as compatible. Hence, the invention is not limited to the described embodiments, but can be varied within the scope of the enclosed claims. In particular, various embodiments separated with "or" in the text may be combined in the claims, in particular various concentrations and ratios of heavy metals. While the invention has been described with reference to specific exemplary embodiments, the description is in general only intended to illustrate the inventive concept and should not be taken as limiting the scope of the invention. The invention is generally defined by the claims.

EXAMPLES

Example 1

A sample of 18.62 kg of biosludge (BS) was obtained from the SCA Ostrand pulp manufacturing plant in Timra, Sweden. The biosludge was obtained from the biological wastewater treatment plant of the pulp manufacturing plant. A sample of fiber sludge (FBS) containing 60% of biosludge and 40% of fiber sludge was produced by mixing biosludge with fiber sludge from the same source. The fiber sludge fraction was isolated from a fraction of wastewater that was collected from the pulping process but had not passed a biological treatment step, hence it was not biosludge. Bacterial analysis

The sludge samples were frozen at -18°C. Before the experiment, each of the sludge were thawed and heated to 100°C -110°C to kill potential bacteria. They were then frozen again and kept at -18°C until further use. However, a small non-heated sample of each sludge sample was kept for bacterial analysis as described below. The amounts of sludge needed for the feeding experiments thawed on a daily basis.

The bacterial analysis was to detect the presence or absence of Legionella, spp and Salmonella. spp. The analysis showed the presence of Salmonella, spp in the non-heated 100% biosludge (BS) sample only. The analysis did not indicate presence of bacteria in other three samples (non-heated fibersludge, heated fibersludge (FBS) and heated biosludge).

Mealworm (Tenebrio molitor) larvae were fed either control feed or two different types of biosludge feeds (BS= Biosludge, FBS=Fiber biosludge).

The growth of the larvae over time is shown in Fig. 2. An increase of the larval weight occurred at the middle of the 3rd week post feeding introduction. The growth of the larvae fed on biosludge (BS) and fiber biosludge (FBS) was comparable to larvae fed control feed. Growth was greatly improved over trials in which biosludge was used as single feed without addition of wheat bran. In general, the larvae looked healthy and was of a healthy size. Example 4

General observations regarding the harvest.

Larvae fed with biosludge (BS) could be harvested in a regular fashion. The larvae fed with the fibersludge (FBS) were more complicated to harvest. Indeed, the larvae fed with FBS extracted the sludge from the fiber, letting those fibrous dust at the surface of the wheat bran. This dust was sticking the larvae and created lot of fibrous balls which agglutinated on the vibrating mesh tray of the harvesting machine, creating a congestion close to the separator and complicating the separation of this substrate from the larvae.

Example 5

The amount of larvae, frass, heavy residues and light residues at harvest (week nine) were quantified, Fig 3. Larvae fed with biosludge (BS) and larvae fed with control feed produced a higher volume of frass and a smaller volume of heavy residues and light residues than larvae fed with fiber sludge (FBS).

Example 6

Chitin content in frass was quantified. The results are shown in table 1.

Table 1.

The chitin content was somewhat higher in the frass from larvae fed biosludge only. Example 7

Similar results as in Example 3, 4 and 5 were obtained when the biosludge was given separate from the wheat bran. The bio sludge was then given as a portion on top of the wheat bran.

Example 8

The concentration of heavy metals in biosludge that was fed to the larvae and in the frass after harvest and separation of living larvae from frass was analyzed. These larvae were not fed fiber biosludge. The results are shown in Table 1.

Heavy metals were determined with the use of ICP-SFMS according to SS EN ISO 17294-1, 2 (mod) and United States EPA method 200.8. Dissolution was carried out in a microwave oven in closed Teflon containers using HNO3 /H2O2/HF. Average values from three triplicates are shown in Table 1.

Table 1.

The results show that the concentration of heavy metals was between 10-200 times higher in the frass as compared to the living larvae. Because the frass represents only a fraction of the harvest (about 45-75 %) and because some of the weight of the starting material is have been lost as carbon dioxide during culturing, the actual enrichment of heavy metals in the frass is significantly higher than the figures shown in Table 1. Material and methods

Maintenance and

of the

treatments The sludge samples were frozen at -

18°C. Before the experiment, each sludge sample was heated to 100°C-110°C to kill potential bacteria and allow to create the needed amount sample bags of each ST which were frozen again and kept at -18°C. However, a small non-heated sample of each sludge sample was kept for bacterial analysis as described below. The sludge amount was thawed day by day for the necessary experiment needs.

Bacterial ana

is A bacterial analysis focusing on Salmonella spp and Legionella spp was made by Micans, Microbial Analytics Sweden AB, Mblnlycke, Sweden (heated and nonheated).

new Tenebrio molitor colony dedicated to R&D trials was generated using the oviposition (eggs stage) stock established 3 weeks upstream within the current mealworm production of Tebrito AB. Mealworms was reared and maintained at 29°C, 60% relative humidity and with a photoperiod of 12H-12H light/darkness. The larvae were cultured in carry units (CU) (56.6x36.6x7.2cm)

Diet Before the trial, the larvae were fed wheat bran (w0-w3) From w4, the larvae were fed a food mixture containing wheat bran mixed (50%-50%) with BS or FBS. This was compared to a control group that were given a feed mixture of wheat bran and potato peals (50%-50%).

Timetable and

BS, FBS or control feed was given to 18 CU:s each The total experiment lasted 9 weeks (W0-W9) from the oviposition (egg stage) to the larval stage and finally the harvest.

W-2 - W-l: An oviposition stock of adult beetles was used to create new batch for the experiment. All the adult beetles from each CU were separated from their eggs laying and transferred into new CUs filled with wheat bran to allow them to lay eggs and create a new progeny. Those adults were fed daily with 50g of carrots.

WO - W3: Creation of a new progeny from the oviposition stock of the Tebrito production: The first day of the experiment, the new progeny was generated from the oviposition stock and fed wheat bran. Then, each CU was watered 3 times a week on the second and the third week after starting the experiment.

W4-W8: Introduction of the test feed and + larval sampling for larval growth reporting: On the Monday of W4, each group of larvae was introduced to test feeds. Every Monday, the larvae a received quantity of one of the test feeds. Those quantities increased each week (W4: 100g, W5-6: 150g; W7-8: 300g). The control group received the same quantities of potato peals mixed with wheat bran (W4: 100g, W5-6: 150g; W7-8: 300g).

From the 3rd (W2) to the 5th (W4) week post-feeding introduction, a minimum of 20 larvae of 5 CU per studied group were sampled and weighted 3 times a week (Monday, Wednesday, Friday) for growth sampling reporting.

W9: Harvest + nutritional analysis: The larvae of each studied group were starved 24h before to undergo the harvesting process. This process allowed to separate the larvae from the frass and from the residues (light and heavy). For this purpose, the harvested larvae and frass were placed on a mesh tray CU overlapped on a CU in order to separate them from frass during 24h. Following this process, the different harvested products (larvae, frass, and residues) were weighted.

Chitin quantification in frass

This step has been made by RISE, the Research Institute of Sweden AB (Boras) in order to quantify the chitin amount within frasses of each trial. For such purpose, a highly selective derivatization method for glucosamine (GlcNH2) in combination with LC/MS was chosen. The method consisted of: to weight each sample and hydrolyse them in 50% HCI during 2h at 100°C. The hydrolysis solution was evaporated, and each sample was dissolved again in a borate buffer after which the free amines were derivatized with benzyl formale chloride dissolved in acetonitrile for lh at 30 °C. The reaction was finally stopped with the addition of a diamine. The content of derivatized GIcNHz was determined by LC / MS against an external standard curve of derivatized GIcNHz. The yield of hydrolysis and derivatization was determined by the addition of pure chitin to each sample before hydrolysis as a reference. As the hydrolysis of polymers often generates many different products, it is important to correct the result towards hydrolysis and the derivatization yield.

Statistical analysis: The statistical analysis of the results has been made with the software GraphPad Prism 9.2.0.

Claims

1. A method for processing biosludge from a paper pulp manufacturing plant comprising the steps of a) providing biosludge from the paper pulp manufacturing plant b) feeding the biosludge to a culture of larvae from the order Coleoptera (beetles).

2. The method according to claim 1 where the biosludge comprises lead at a concentration of at least 1 mg/kg dry substance, or cadmium at a concentration of at least 0.1 mg/kg dry substance, or mercury at a concentration of at least 0.01 mg/kg dry substance or copper at a concentration of at least 1 mg/kg dry substance or arsenic at a concentration of at least 0.25 mg/kg dry substance.

3. The method according to claim 1 where the biosludge comprises lead at a concentration of from 1 mg/kg dry substance to 100 mg/kg dry substance, or cadmium at a concentration of from 0.1 mg/kg dry substance to 15 mg/kg dry substance, or mercury at a concentration of from 0.01 mg/kg dry substance to 0.5 mg/kg dry substance, or copper at a concentration of from 1 mg/kg dry substance to 100 mg/kg dry substance or arsenic at a concentration of from 0.25 mg/kg to 10 mg/kg dry substance.

4. The method according to any one of claims 1 to 3 where the method comprises the step of, at the end of a culturing period, separating frass and the larvae as separate fractions from the culture, and where the concentration of a heavy metal in the larvae is at most: 0.001 mg lead/kg dry substance or 0.001 mg cadmium/kg dry substance or 0.001 mg mercury/kg dry substance or 0.01 mg copper/kg dry substance or 0.001 mg arsenic/kg dry substance. The method according to an any one of claims 1 to 4 where the method comprises the step of, at the end of a culturing period separating frass and the larvae as separate fractions from the culture, and where the ratio, of the concentration of a heavy metal in the larvae to the concentration of the heavy metal in the frass, is at most 0.25 for one of lead, cadmium, mercury, copper or arsenic. The method according to claim 1 to 4 where the method comprises the step of, at the end of a culturing period, separating frass and the larvae as separate fractions from the culture, and where the ratio, of the concentration of a heavy metal in the larvae to the concentration of the heavy metal in the frass, is from 0.001 to 0.25 for lead or from 0.002 to 0.25 for cadmium or from 0.01 to 0.25 to for mercury or from 0.001 to 0.25 for copper or from 0.005 to 0.25 for arsenic. The method of any one of claims 1 to 6 where the larvae is from a species selected from Tenebrio molitor (mealworm beetle), Zophobas atratus (Giant mealworm), Alphitobius diaperinus (Lesser mealworm, buffalo worm), Pachnoda butana (Sun beetle), Pachnoda aemole (Sun beetle), Pachnoda marginata (Sun beetle), Zophobas morio (King worm, greater mealworm, Superworm, Morio), Sitophilus granarius (Wheat weevil), Sitophilus oryzae (Rice weevil), and Oryctes nasicornis (European rhinoceros beetle). The method of any one of claims 1 to 6 where the larvae is from Tenebrio molitor (mealworm beetle). The method of any one of claims 1 to 8 where the biosludge comprises from 10 % to 40 % cellulose (dry substance) and from 10 % to 50 % lignin (dry substance). The method of any one of claims 1 to 9 where the larvae is additionally fed a plantbased material. A system for processing biosludge from a paper pulp manufacturing plant comprising a paper pulp manufacturing plant comprising a biological wastewater treatment plant and a wastewater outlet and biosludge separation means for separating biosludge from the wastewater outlet, the system further comprising an insect culturing facility comprising a culture of larvae from the order Coleop- tera (beetles). The system of claim 11 further comprising transportation means for transporting the biosludge from the biosludge separation means to the insect culturing facility. The system of claim 12 where the transportation means is a conveyor belt, a screw or a pipeline. The system of any one of claims 11 to 13 further comprising a biosludge heater.