NL2009007C2

NL2009007C2 - Improved treatment of sludge.

Info

Publication number: NL2009007C2
Application number: NL2009007A
Authority: NL
Inventors: Antonius Theodorus Wilhelminus Maria Hendriks
Original assignee: Antonius Theodorus Wilhelminus Maria Hendriks
Priority date: 2012-06-14
Filing date: 2012-06-14
Publication date: 2013-12-17

Description

Improved Treatment of sludge

FIELD OF THE INVENTION

The present invention is in the field of treating sludge, in 5 particular sewage sludge, waste water sludge and the like, as well as similar mixtures, especially in view of components therein comprised.

BACKGROUND OF THE INVENTION

In general background of treatment of sludge is known 10 to a person skilled in the art.

In the present invention sludge refers to the residual, semi-solid material left from industrial wastewater, or sewage treatment processes. It can also refer to the settled suspension obtained from conventional drinking water treat-15 ment, and numerous other industrial processes. The term may also sometimes be used as a generic term for solids separated from suspension in a liguid. Typically two types of sludge are considered, one having a relative high solids content (> 7% w/w) and one having a relative low solids content (<7% w/w).

20 When fresh sewage or wastewater is added to a set tling tank, typically approximately 50% of the suspended solid matter will settle out typically in an hour and a half. The sludge will then become putrescent in a short time once anaerobic bacteria take over, and may preferably be removed from 25 the sedimentation tank before this happens. It is noted that at this stage preferably no anoxic and anaerobic processes occur .

Removal can be accomplished in various ways. In an Imhoff tank, fresh sludge may be passed through a slot to the 30 lower story or digestion chamber where it is decomposed by anaerobic bacteria, resulting in liquefaction and reduced volume of the sludge. After digesting for an extended period, the result typically is called "digested" sludge and may be disposed of by drying and then land filling. More commonly with domes-35 tic sewage, the fresh sludge may be continuously extracted from the tank mechanically and passed to separate sludge digestion tanks that operate at higher temperatures than the lower story of the Imhoff tank and, as a result, digest much more rapidly and efficiently.

40 Excess solids from biological processes such as acti- 2 vated sludge may still be referred to as sludge, but the term bio solids, is more commonly used. Surface water plants also generate sludge made up of solids removed from the raw water.

Sewage sludge can be produced from the treatment of 5 wastewater and typically consists of two basic forms — raw primary sludge (basically faecal material) and secondary sludge (a living 'culture' of organisms that help remove contaminants from wastewater before the water may be returned to rivers or the sea). The sludge typically is transformed into 10 bio solids using a number of complex treatments such as digestion, thickening, dewatering, drying, and lime/alkaline stabilisation .

Sludge can be recycled in a variety of ways. These include using anaerobic digestion to produce biogas, pyrolysis 15 of the sludge to create syngas and potentially biochar, or incineration in a waste-to-energy facility for direct production of electricity and steam for district heating or industrial uses. If methane is captured rather than allowed to outgas, it can be used for fuel, closing the carbon cycle.

20 Sewage treatment generally involves three stages, called primary, secondary and tertiary treatment. It is noted that often stages are combined, either implicitly (such as when in a first stage (an(aerobic)) digestion already is started) or explicitly. It is noted that sludge may vary in 25 composition in time, per location, per climate condition, etc., i.e. it is typically never exact the same. Sludge typically relates to a chemical and biological complex composition and is therefore as such difficult to control. Sludge treatment needs to address the above variations.

30 Primary treatment may consist of temporarily holding the sewage in a quiescent basin where heavy solids can settle to the bottom while oil, grease and lighter solids float to the surface. The settled and floating materials may be removed and the remaining liquid may be discharged or subjected to 35 secondary treatment.

Secondary treatment typically removes dissolved and suspended biological matter. Secondary treatment is typically performed by indigenous, water-borne micro-organisms in a managed habitat. Secondary treatment may require a separation 40 process to remove the micro-organisms from the treated water 3 prior to discharge or tertiary treatment.

Tertiary treatment is sometimes defined as anything more than primary and secondary treatment.

Typically sludge may comprises a significant amount 5 of organic material and humus like compounds.

In soil science, humus refers to any organic matter that has reached a point of stability, where it will break down no further.

Plant remains contain organic compounds such as sug-10 ars, starches, proteins, carbohydrates, lignins, waxes, resins, and organic acids. Organic matter decay in the soil may be regarded as a step wise process. For instance, lignin, which may be transformed by white-rot fungi, is one of the main precursors of humus, together with by-products of micro-15 bial and animal activity. The humus, that is the end-product of this manifold process, is thus a mixture of compounds and complex life chemicals of plant, animal, or microbial origin, which has many functions and benefits in the soil.

Typically a mixture further comprises cations and 20 anions. As in soil part of these ions, in particular the cations, are often selectively bound to other components being present, such as humus.

The concept of selectivity is used to quantify the extent to which a given substrate, A, binds two different binding 25 sites, B and C.

It is noted that various components of e.g. sludge are not easily converted, such as humus. Such is partly due to humus being not very accessible to microorganisms.

Also humus and humus like compounds are regarded det-30 rimental for various other processes, such as digestion by microorganisms, e.g. the amount of material converted and thus yields are limited.

Prior art treatment methods suffer from a limited conversion of sludge, in particular of organic material being 35 present therein. For instance, the amount of energy generated, such as by combustion or potential energy (gas) when digestion, is quite limited, e.g. to about 25%-40%.

Also the amount of primary and secondary sludge that remains after processing thereof is relatively large. This 40 amount puts a burden on the environment.

4

Further, various pre-treatment methods exist. A first type of pre-treatment is performed at relatively high temperatures, typically higher than 90 °C, such as above 121 °C, which may be referred to as thermal pretreatment. An important 5 aspect of such treatments are to kill most or all bacteria being present in sludge. Such pre-treatment, especially at higher temperatures, needs to be performed under pressure, e.g. in an autoclave. As a consequence costs are relatively high.

10 Typically pre-treatment is also performed over a relatively long period of e.g. more than one day.

Often conditions of pretreatment are relatively harsh, e.g. by increasing or decreasing the pH to a value above 8, typically 9 or more, or below 6, typically below 5.

15 Typically a higher pH, e.g. above 9 or 10, or a lower pH, e.g.

below 6 or even below 5, is considered better, i.e. (relative) harsh conditions. Such is in view of the amount of chemicals needed not very cost effective, even if as a result thereof in a later stage yield of digested products, e.g. methane, is in-20 creased. It is also noted that typically the pH thereafter needs to be readjusted to more or less neutral values, involv ing further chemicals, in order to obtain satisfactory conversion yields.

Sometimes further additional measures are taken, such 25 as addition of chemicals, such as ozone, peroxides, etc., ultrasone treatment, addition of steam, mechanical action, etc.

Low temperature pre-treatment are considered as well. Typically very long times are involved, such as more than 1 day, typically up to 7 days. For such treatment temperature 30 ranges are typically considered and tested, resulting, as above, in a high temperature and/or a long treatment time.

A general conclusion of the prior art is that pretreatment is preferably performed at high temperature (the higher the better) and over a long period of time (the longer 35 the better). The higher the temperature the shorter the treatment may be. Of course there is a point where no further improvement is observed.

As mentioned above sometimes a combination of treatment stages is considered, e.g. by (biologically) enriching a 40 sludge at the corresponding increased temperature with bacte- 5 ria capable of (an(aerobic)) converting (or digesting) the sludge. As a consequence a preferred temperature of pretreatment would then be a preferred temperature of conversion, such as a thermophilic or mesophilic temperature. Such a pre-5 treatment could be considered a thermal biological pretreatment .

It is also considered to separate various components of sludge, e.g. water and solids, such as by using a membrane. For limited applications this may be beneficial, but in gen-10 eral it is not.

The present invention therefore relates to a method for treatment of sludge, preferably untreated sludge, in particular sewage sludge, waste water sludge and the like, and similar mixtures, especially in view of the components therein 15 comprised, which overcomes one or more of the above disadvantages, without jeopardizing functionality and advantages.

SUMMARY OF THE INVENTION

The present invention relates in a first aspect to a method of treating a mixture according to claim 1.

20 As such the method can be considered to relate to a low temperature (pre)treatment method. It is noted that no mechanical (pretreatment) measures are considered to be involved in the present method, apart from e.g. pumping. Further, apart from an optional exchange of Ca-ions, no chemicals are in-25 volved in the thermal treatment step. Apart from an increase of temperature the present method is preferably performed at ambient conditions (e.g. pressure, air) and at conditions wherein sludge or the like is obtained (e.g. pH, biological status, solids content), e.g. no bacterial culture is added to 30 the mixture. As a consequence e.g. no or almost no methane is formed in the present temperature step. Such is advantageous as a conversion step potentially yields more methane, as well as the initial formed methane is typically lost and forms a greenhouse gas. In view of complexity of e.g. an installation 35 such is also advantageous.

The present invention is particularly suited for sludge having a relative low solids content (<12% w/w, such as from 5-8%w/w, often from 5-6,5% w/w).

Typically the present invention is aimed at treatment 40 of biomass, such as sludge, obtained after the above mentioned 6 first treatment stage, namely after settlement of sludge etc. i.e. secondary sludge. Sludge may therein be obtained from waste water.

The present method provides for a solution allowing 5 for a significant increase in conversion, thereby generating amongst others an increase in (potential) energy obtained.

Even further, the amount of waste sludge is decreased significantly. Also the waste sludge is of a higher quality.

The present invention provides for a relative com-10 plete destruction and/or perforation of cell membranes of bacteria, biomass and the like. Such could be regarded as a thermal treatment. Otherwise the treatment is mainly of a chemical nature. Thereto a combination of the temperature, being high enough, and the time, being be long enough, should be pro-15 vided. It has been found experimentally in this respect that at least a temperature of 60 °C should be used.

The present invention is also aimed at preventing or at least reducing (one or more) chemical side products, as these side product are unwanted, e.g. in that they are detri-20 mental to further processing, such as conversion. In this respect especially condensation reactions are prevented or reduced. It has been found experimentally in this respect that at most a temperature of 80 °C should be used. Such is also envisaged from an economical point of view.

25 In order to establish a practical relation between temperature and time it is considered that effectively an amount of energy is provided. Such is in line with consideration e.g. in view of pasteurisation or sterilisation. Such consideration consider time to be of a linear nature and tem-30 perature to be of an exponential nature. Such is further in line with what inventors call a thermal budget indicated with Et,Tf wherein the thermal budget is. taken to be equal to Et,T -t*exp (0,15*(T-65 °C) ) , wherein T is the temperature (T, °C) and t is a period of time (t, min). It has been found experi-35 mentally that best results, e.g. in terms of yield of the subsequent conversion step, are obtained at a temperature of (about) 65 °C. Therefore the thermal budget is "centred" using the above formula around this temperature. The constant 0,15 is chosen such that at the preferred temperature the time is 40 directly obtainable in minutes, and such that reasonable val- 7 ues for Et,x are obtained. It has further been found that time may be relative short, such as 5 minutes, providing acceptable results, e.g. destruction and/or perforation of cell membranes. Likewise times may be relatively long, such as 1440 5 minutes; however it is observed experimentally that such longer times do not provide much better results. In this respect 1000 minutes is already considered quite long. A down side of longer processing times is that relatively (and in ratio) larger facilities need to be used, in order to process a 10 same amount of biomass. Such is not cost effective. For practical reasons throughput (or processing) times of less than 120 minutes are preferred. In fact, if times are to long e.g. negative side products are formed, so yield in subsequent conversion step drops. In summary, the present formula provides a 15 very acceptable and practical formula which can be put into use directly.

The thermal budget is provided in a first reaction environment, e.g. a first reactor, whereas (biological) conversion takes place in a second reaction environment, e.g. a 20 second reactor, the first and second environment not being the same. Of course for practical purpose the first environment may be coupled to the second.

The present method provides a biomass that is subsequently converted most efficiently, taking into account biological aspect relevant for microorganisms, e.g. use of energy and nutrients and effects of toxicants, as well as chemistry of the conversion process, e.g. reduction of detrimental side products .

The present method is applicable to sludge and similar mixtures, in particular to secondary sludge.

25 Typically the mixture comprises various compounds, as indicated above. Organic material may at least partly relate to products, such as originating from plants and/or animals.

Typically also humus is present. As described above humus is by itself not a well defined product as it may con-30 sist out of various components, which also depends on the stage of decay.

It is noted that the present invention is also applicable to similar mixtures, not being sludge, such as organic material comprising mixtures, such as manure, grass, etc.

8

Conversion may in principal relate to any physical, chemical, biological, micro biological, and combinations thereof, conversion.

If the conversion involves production of e.g. a fuel, 5 the fuel can be used to heat the mixture.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates in a first aspect to a method of treating a mixture as indicated throughout the description.

10 In an example of the present method the biomass com prises cell material, and/or wherein the carbohydrates comprise one or more monosaccharides, disaccharides, oligosaccharides, and polysaccharides, and/or wherein the humus comprises one or more of fulvic acids, humine, humic acids, such as hy-15 matomel acid, grey and brown humic acids.

In the example various forms of carbon, hydrogen and oxygen containing compounds are present.

In an example the thermal budget is from 12-300 [°C*min], preferably from 15-200 [°C*min], more preferably 20 from 20-100 [°C*min], even more preferably from 25-80 [°C*min], such as from 30-60 [°C*min]. In an example a temperature of 65 °C and a time of 60 minutes provide excellent results. In other words the thermal budget is relativity limited. Surprisingly, contrary to teachings of the prior art, a 25 limited thermal budget is found to be enough, e.g. in terms of conversion, chemistry and biology of a mixture obtained. It has been found that a relative moderate temperature and a relative short time, as detailed below, result in the highest yields .

30 It has been found experimentally that when the Ca2+ is exchanged and preferably made unavailable the organic material can be converted subsequently with higher efficiency. Preferably such a conversion is further supported by addition of nutrients in case microorganisms are used, in order to further 35 increase yield, such as by addition of carbohydrates in suitable amounts.

It is noted that grey humic acids can be deposited by adding an electrolyte, whereas brown humic acids can not. Further, also hymatomel acid is considered a humic acid.

40 It has been found that by exchanging a substantial 9 part of the Ca2+ and by a subsequent temperature treatment the conversion of the mixture into e.g. biogas is improved significantly, thereby increasing yield by 25-100% relatively, depending on boundary conditions, such as type of mixture. Al-5 ternatively, though less preferred, heating and exchanging can be performed more or less at the same time, such as in one process step.

In an example the present invention is aimed at exchanging the Ca2+ such as exchanging by another cation. It is 10 preferred that the other cation has a similar selectivity and behaviour as the Ca2+has, e.g. in terms of size, including a possible water shell surrounding the cation, binding capacity, selectivity towards other compounds, such as humus, etc.

It is noted that also a combination of cations and 15 anions may be used.

In an example the Ca2+ is made unavailable, e.g. by forming a salt and/or complex thereof. The chemical equilibrium of the exchange process is thereby shifted towards (total) removal of the calcium. As such less exchangeable cation 20 is required. The mixture may have a lower salt/complex (Ca2+) content as a consequence. A lower content may improve conversion, and also is less detrimental to the environment.

Such also indicates that the other cation preferably has a solubility with the anion which may form a salt with the 25 calcium which is significantly larger than that of calcium and the anion, preferably at least an order larger. Of course a combination of salts may be provided, such as NaCl and Na2C03.

A similar reasoning is applicable if a complex forming agent, such as EDTA, is added.

30 The person skilled in the art can determine suitable combinations, taking into account boundary conditions.

After the calcium has been exchanged and preferably been made unavailable the temperature of the mixture is increased, such as by heating. The heating and as mentioned 35 above the calcium exchange improve the subsequent conversion of the mixture.

In an example of the present method the anion is selected from the group comprising C032-, S042", OH", carboxylic acid ions, P043", HP042~, H2P04~, S2~, HS", Cl", HC03", N03", and 40 combinations thereof.

10

In a further example an anion is chosen that does not change the pH too much. A final pH is preferably in the range of 4-10, more preferably from 5-9, even more preferably between 5-8, such as from 6-7.0. Therefore (small amounts of) 5 HCO3", C032" and carboxylic acid ions, such as acetic acid, COOH, etc., are preferred.

It is also preferred that the anion forms a low soluble salt with Ca2+. Such anions are readily available.

In an example of the present method the cation is se-10 lected from the group comprising Na+, Mg2+, Fe2+, Fe3+, Al3+, K+, Cu2+, Zn2+, NH4+, H+, and combinations thereof.

Preferably the cation forms a soluble salt with the anion. Therefore Na+, NH4+, H+, and K+, are preferred.

Example of salts are NaCl, NaHC03, Na2C03, (NH4)2C03, 15 FeC03, FeCl2, NH4HC03, KC1, KHC03, K2C03, NaAc, KAc, and NH4Ac.

In an example of the present method the complex forming agent is selected from the group comprising EDTA, complex forming acids, complex forming polymers, such as poly(meth)acrylic acid, and combinations thereof.

20 Examples are Na2EDTA, (NH4)2EDTA and FeEDTA.

Preferably the salt and/or complex are relatively environmentally friendly, and/or are biodegradable.

Also a combination of a salt and complex may be used, such as (NH4)2EDTA and NaHC03.

25 In an example of the present method the amount of ex changeable Ca2+ is determined prior to exchanging.

It is advantageous to determine the amount of Ca first, as thereafter the amount of ions to be added can be tuned. A method to determine the amount of exchangeable cal-30 cium, albeit for a different purpose, is described below.

In an example of the present method the amount of anion added is sufficient to exchange at least 66% of the amount of exchangeable Ca2+ being present, preferably to exchange at least 90% of the amount of exchangeable Ca2+, more preferably 35 to exchange at least 95% of the amount of exchangeable Ca2+, even more preferably to exchange at least 99% of the amount of exchangeable Ca2+, such as at least 99,9% of the amount of exchangeable Ca2+.

It has been found that in order to improve conversion 40 significantly the amount of exchanged Ca2+ is substantial, i.e.

11 at least 50% thereof. Even better results are obtained if the amount of exchanged Ca2+ is increased further, whereas the best results are obtained is substantially all the Ca2+ is exchanged. Such will however depend on the nature of the mix-5 ture, and boundary conditions. Also typically a cost effective approach will be taken.

As mentioned above the exchange process can be improved by making the calcium unavailable.

In an example temperature is increased to 61-75 °C, 10 preferably to 62-70 °C, more preferably to 63-68 °C, preferably at ambient pressure. An optimal temperature in view of the present objectives has been found to be slightly above 60 °C, in order to e.g. destruct and/or perforate cell membranes, and below 70 °C, in order to prevent e.g. adverse side products.

15 As mentioned an optimal temperature is close to 65 °C, which temperature may vary somewhat (e.g. 63-70 °C) depending on e.g. nature of the biomass. It is therefore not preferred to move to substantial higher temperatures, as suggested by the prior art.

20 Preferably the temperature is not too high, in view of costs and in view of regulations. On the other hand the temperature is preferably not too low, as than a smaller effect is obtained, e.g. in terms of extra conversion and yield obtained.

25 Also the conversion rate is improved by the present process. Depending on the nature of the mixture the conversion rate is improved by 5-100%, i.e. somewhat faster up to about two times faster. Such implies amongst others a shorter throughput time, or likewise an increased capacity.

30 In an example of the present method the temperature increase is performed during a period of 6-720 minutes, preferably 10-240 minutes, such as 20-60 minutes. Surprisingly, at the present relatively low temperatures, also a relatively short time may be chosen. A time of 30-45 minutes is typically 35 enough.

It has been found that a relative short period provides for the best results. Longer times may be used, improving the conversion even further, but this is considered less cost effective. Also too long times are detrimental, e.g. due 40 to formation of side products. A too short time does not pro- 12 vide much improvement.

In an example of the present method the converting is a biological conversion, such as digestion, such as anaerobic or aerobic digestion, and combinations thereof. Thereby e.g.

5 biogas, hydrogen or ethanol is formed. Preferably an anaerobic process is used, as presence of oxygen and/or oxidising compounds reduce the yield.

The invention is further detailed by the accompanying examples, which are exemplary and explanatory of na-10 ture and are not limiting the scope of the invention. To the person skilled in the art it may be clear that many variants, being obvious or not, may be conceivable falling within the scope of protection, defined by the present claims .

15 Experiments

Experimental Setup

Substrate

As substrate secondary waste activated sludge (SWAS) from a wastewater treatment plant (WWTP) is taken. The amount of 20 mixed liquor volatile suspended solids (MLVSS) is determined and expressed as gMLVSS/1.

Example 1

Thickened SWAS is taken and heated to 60, 65, 70, 75 and 80°C, for, in an the example, 10 minutes. After heating 25 during the predetermined time the substrate is submitted to a biological conversion, in the example, anaerobic digestion. In a sub-example, during the anaerobic digestion methane is produced from the substrate, with a sludge retention time of 20 days. It is noted that for mesophilic 30 digestion retention times are typically in the order of 18-30 days, such as 22-24 days, and for thermophilic digestion retention times are typically in the order of 12-20 days, such as 14-16 days. The untreated substrate has, in a prior art example, a conversion efficiency of 25-40%, depending 35 on conditions and material chosen, while the pre-treated material of the present example has a conversion efficiency of 35-75% (not detailed below), depending on conditions and material chosen, i.e. a surprising high increase of 10%-50% absolute. Such depends somewhat on the nature of sludge, 40 the amount of organic material therein, the nature of or- 13 ganic material, etc.

Table 1: Pretreatment time 10 minutes_

Pretreatment °C Untreated 60 65 70 75 80 temperature_____

Methane mlCH4/gVS 164 174 258 263 244 199 production_____ VS reduction %__33__35__52_ 54 49 40 5 Example 2

Thickened SWAS is taken and heated to 60, 65, 70, 75 and 80°C, for, in the example, 30 minutes. After heating during the predetermined time the substrate is submitted to a biological conversion, in the example, anaerobic digestion. In 10 a sub-example, during the anaerobic digestion methane is produced from the substrate, with a sludge retention time of 20 days. The untreated substrate has, in a prior art example, a conversion efficiency of 25-40%, while the pretreated material of the present example has a conversion 15 efficiency of 32-79% (not detailed below), i.e. an increase of 7%-54% absolute. Such depends somewhat on the nature of sludge, the amount of organic material therein, the nature of organic material, etc.

20 Table 2: Pretreatment time 30 minutes _____

Pretreatment °C Untreated 60 65 70 75 80 temperature________

Methane mlCH4/gVS 164 174 258 263 244 199 production_________ VS reduction %_[_27_ 32 58 [48 41 35

Example 3

Thickened SWAS is taken and heated to 60, 65, 70, 75 and 80°C, for, in the example, 60 minutes. After heating during 25 the predetermined time the substrate is submitted to a biological conversion, in the example, anaerobic digestion. In a sub-example, during the anaerobic digestion methane is produced from the substrate, with a sludge retention time of 20 days. The untreated substrate has, in a prior art ex-30 ample, a conversion efficiency of 25-40%, while the pre- 14 treated material of the present example has a conversion efficiency of 42-82% (not detailed below), i.e. an increase of 17%-57% absolute. Such depends somewhat on the nature of sludge, the amount of organic material therein, the nature 5 of organic material, etc.

Table 3: Pretreatment time 60 minutes ___ __

Pretreatment °C Untreated 60 65 70 75 80 temperature

Methane mlCH4/gVS 164 174 258 263 244 199 production VS reduction %_|_39_ 42 [54 47 44 43

Example 4 10 Thickened SWAS is taken and heated to 60, 65, 70, 75 and 80°C, for, in the example, 120 minutes. After heating during the predetermined time the substrate is submitted to a biological conversion, in the example, anaerobic digestion. In a sub-example, during the anaerobic digestion methane is 15 produced from the substrate, with a sludge retention time of 20 days. The untreated substrate has, in a prior art example, a conversion efficiency of 25-40%, while the pretreated material of the present example has a conversion efficiency of 37-74%(not detailed below), i.e. an increase 20 of 12%-49% absolute. Such depends somewhat on the nature of sludge, the amount of organic material therein, the nature of organic material, etc.

Table 4: Pretreatment time 120 minutes _____

Pretreatment °C Untreated 60 65 70 75 80 temperature __

Methane mlCH4/gVS 164 174 258 263 244 199 production VS reduction %__32_ 37 56 [39 [39 38 25

Example 5

Thickened SWAS is taken and the concentration MLVSS is determined. After this, the amount of exchangeable calcium is determined as described. After determination of the amount 30 of exchangeable calcium another cation, which replaces the 15 calcium cation, is added in excess. In the example, the amount of exchangeable calcium is 0.3 mmol/gMLVSS and the MLVSS concentration is 50 g/1. Therefore at least >15 mmol/1 of, in the example, sodium, is added. To ensure a 5 large part of the exchangeable calcium is replaced the amount of sodium added is, in the example, chosen to be a factor 2 greater, thus 30 mmol/1. The sodium is preferably added as a salt of which the anion forms an insoluble salt with the calcium, in the example sodium carbonate, or forms 10 chelates with the calcium, in a further example sodium-EDTA.

After adding the salt and mixing, the mixture is heated to, about 65°C, for, in the example, 30 minutes. After heating during the predetermined time the substrate is submitted to 15 a biological conversion, in the example, anaerobic digestion with a sludge retention time of 20 days. In a subexample, during the anaerobic digestion methane is produced from the substrate. The untreated substrate has, in a prior art example, a conversion efficiency of 30-40%, while the 20 pre-treated material of the present example has a conversion efficiency of 45-80%, i.e. an in-crease of 5%-50% absolute. Such depends somewhat on the nature of sludge, the amount of organic material therein, the nature of organic material, the amount of Ca2+, etc.

25 Determination of exchangeable calcium amount A method for determining exchangeable calcium in activated sludge floes is for instance disclosed in B. Peeters et al. in Separation and Purification Technology, (2011) (accepted paper).

30 The SWAS is divided into two egual parts. The pH

of the first part of the SWAS is adjusted to 4 by addition of hydrochloric acid (HC1).

Aluminiumchloride (A1C13) is added at the second part of the SWAS at a dosage of about 12 meq Al3+/g MLVSS. After 35 aluminiumchloride addition also the pH of the second part is adjusted to 4. Both the samples are mixed for about 5 hours (at room temperature). After 5 hours the amount of calcium in the solution is measured. This measurement can for example be done by using inductive coupled plasma - op-40 tical emission spectrometry (ICP - OES). The difference in 16 calcium amount is the amount of exchangeable calcium. By knowing the amount of exchangeable calcium and the amount of MLVSS the amount of exchangeable calcium per gram MLVSS can be calculated.

5 Pre-treatment

After determination of the amount of exchangeable calcium, the amount of cation addition can be calculated. To replace most of the exchangeable calcium the cation that replaces the calcium is typically added in (some) excess. After e.g. 10 adding the replacing cation in excess the mixture is heated for a certain time. After the pre-treatment the substrate can be submitted to any biological conversion. At present digestion by microorganisms is performed with a sludge retention time of 20 days, thereby forming biogas.

15 Example 6: sodium carbonate

Thickened SWAS is taken and the concentration MLVSS is determined. After this, the amount of exchangeable calcium is determined as described above. After determination of the amount of exchangeable calcium another cation, which re-20 places the calcium cation, is added in excess. If, in an example, the amount of exchangeable calcium is 0.3 mmol/gMLVSS and the MLVSS concentration is 50 g/1, at least >15 mmol/1 of, for example, sodium, is added. To make sure a large part of the exchangeable calcium is replaced the 25 amount of sodium added is, for example, chosen a factor 4 greater, thus 60 mmol/1. The sodium is preferably added as a salt of which the anion forms an insoluble salt with the calcium, in an example sodium carbonate, or forms chelates with the calcium, in a further example sodium-EDTA.

30 After adding the salt and mixing, the mixture is heated to, about 80°C, for, in an example, 30 minutes. After the heating for the predetermined time the substrate can be submitted to any biological conversion, like, for example, anaerobic digestion with a sludge retention time of 20 days. 35 In an example, during the anaerobic digestion methane is produced from the substrate. The untreated substrate has, in a prior art example, a conversion efficiency of 30-40%, while the pre-treated material of the present example has a conversion efficiency of 50-80%, i.e. an increase of 10%-40 40% absolute. Such depends somewhat on the nature of 17 sludge, the amount of organic material therein, the nature of organic material, the amount of Ca2+, etc.

Example 7: Ferrous Iron Chloride Thickened SWAS is taken and the concentration MLVSS is de-5 termined. After this, the amount of exchangeable calcium is determined as described above. After determination of the amount of exchangeable calcium another cation, which replaces the calcium cation, is added in excess. If, in an example, the amount of exchangeable calcium is 0.4 10 mmol/gMLVSS and the MLVSS concentration is 40 g/1, at least >16 mmol/1 of, in an example, ferrous iron, is added. To make sure a large part of the exchangeable calcium is replaced the amount of ferrous iron added is, in the example, a factor 1.5 greater, thus 24 mmol/1. The ferrous iron is 15 preferably added as a salt of which the anion forms an insoluble salt with the calcium, in an example ferrous iron bicarbonate, or forms chelates with the calcium, like ferrous iron-EDTA.

After adding the salt and mixing, the mixture is heated to, 20 about 120°C, for, in an example, during 15 minutes. After the heating for the predetermined time the substrate can be submitted to any biological conversion, like, in an example, anaerobic digestion with a sludge retention time of 20 days. During the anaerobic digestion methane is produced 25 from the substrate. The untreated substrate has, in a prior art example, a conversion efficiency of 20-30%, while the pre-treated material of the present example has a conversion efficiency of 30-65%, i.e. an increase of 10%-35% absolute .

30 Example 8: Potassium Bicarbonate

Thickened SWAS is taken and the concentration MLVSS is determined. After this, the amount of exchangeable calcium is determined as described above. After determination of the amount of exchangeable calcium another cation, which re-35 places the calcium cation, is added in excess. If, in an example, the amount of exchangeable calcium is 0.25 mmol/gMLVSS and the MLVSS concentration is 30 g/1, at least >7.5 mmol/1 of, in an example, potassium, should be added. To make sure a large part of the exchangeable calcium is 40 replaced the amount of potassium added is, in an example, a 18 factor 3.0 greater, thus 22.5 mmol/1. The potassium is preferably added as a salt of which the anion forms an insoluble salt with the calcium, in an example potassium bicarbonate, or forms chelates with the calcium, like potas-5 sium-EDTA.

After adding the salt and mixing, the mixture is heated to, about 100°C, for, in an example, during 20 minutes. After the heating for the predetermined time the substrate can be submitted to any biological conversion, like, in an exam-10 pie, anaerobic digestion with a sludge retention time of 20 days. During the anaerobic digestion methane is produced from the substrate. The untreated substrate has, in a prior art example, a conversion efficiency of 25-35%, while the pre-treated material will than have a conversion efficiency 15 of 50-75%, i.e. an increase of 15%-40% absolute.

Example 9: Sodium EDTA

Thickened SWAS is taken and the concentration MLVSS is determined. After this, the amount of exchangeable calcium is determined as described above. After determination of the 20 amount of exchangeable calcium another cation, which replaces the calcium cation, is added in excess. If, in an example, the amount of exchangeable calcium is 0.3 mmol/gMLVSS and the MLVSS concentration is 50 g/1, at least >15 mmol/1 of, in an example, sodium, should be added. To 25 make sure a large part of the exchangeable calcium is replaced the amount of sodium added is, in an example, a factor 4.0 greater, thus 60 mmol/1. The sodium is preferably added as a salt of which the anion chelates with the calcium, like sodium-EDTA.

30 After adding the salt and mixing, the mixture is heated to, about 150°C, for, in an example, during 10 minutes. After the heating for the predetermined time the substrate can be submitted to any biological conversion, like, in an example, anaerobic digestion with a sludge retention time of 20 35 days. During the anaerobic digestion methane is produced from the substrate. The untreated substrate has, in a prior art example, a conversion efficiency of 15-25%, while the pre-treated material will than have a conversion efficiency of 25-60%, i.e. an increase of 10%-35% absolute.

40 It should be appreciated that for commercial application it 19 may be preferable to use one or more variations of the present system, which would similar be to the ones disclosed in the present application and are within the spirit of the invention.

Claims

A method for treating a mixture per batch, semi-continuous, or continuous, comprising the steps of: - providing in a first reaction environment the biologically non-enriched mixture, such as sludge, which mixture is 2-30% has solid, comprising organic material, such as one or more of biomass, one or more or proteins and enzymes, optionally one or more hydrocarbons, water, and humus, providing a thermal budget Et, t-t * exp ( 0.15 * (T-65 ° C)) to the mixture by maintaining the temperature (T, ° C) for a period (t, min), under conditions such that (i) the thermal budget Et, T is from 10-360 [min * ° C], (ii) the temperature is from 60 ° C - 80 ° C, (iii) the temperature is maintained for a period of 5-1000 minutes, 2 0 (iv) the pH is maintained at a value of between 5-8, more preferably 5.3-7.8, such as 5.5-7.5, and (v) preferably at a pressure of 850-1100 kPa, more preferably of 950-1050, as with oxogevingsdr and converting the mixture to a second reaction environment in a subsequent step.

Method according to claim 1, wherein the thermal budget is from 12-300 [° C * min], more preferably from 15-200 [° C * min], even more preferably from 20-100 [° C * min], even more more preferably from 25-80 [° C * min], such as from 30-60 [° C * min].

A method according to claim 1 or 2, wherein the mixture comprises an exchangeable amount of Ca 2+ of 0.05-10 mmol / g of solid, further comprising the step of exchanging the Ca 2+ and preferably making the Ca 2+ largely unavailable by adding at least one suitable compound such as by adding (i) a salt in a sufficient amount thereof, comprising an anion, which anion preferably forms a poorly soluble salt with Ca 2+, and a cation, wherein the cation and anion preferably have a solubility of at least an order greater than that of the Ca 2+ and the anion, and / or (ii) a Ca 2+ complexing agent in a sufficient amount thereof, which complexing agent comprises a strong complex with Ca 2+ and a cation, the cation preferably forming a weak complex with the agent of at least an order smaller than that of Ca 2+ and the complexing agent.

A method according to any one of the preceding claims, wherein the biomass comprises cell material, and / or wherein the hydrocarbons comprise one or more of monosaccharides, disaccharides, oligosaccharides, and polysaccharides, and / or wherein the humus comprises one or more or fulvic acids, humine , humic acids, such as hymatomic acid, gray and brown humic acid, and humid-like structures.

The method of any one of the preceding claims, wherein the anion is selected from the group comprising CO32 ",

SO42 ", OH", carboxylic acid ions, PO43 ", HPO42", H2PO4 ", S2", HS ", Cl", HCO3 ", NO3", and combinations thereof, and / or wherein the cation is selected from the group comprising Na + , Mg 2+, Fe 2+, Fe 3+, Al 3+, K +, Cu 2+, Zn 2+, NH 4 +, H +, and combinations thereof, and / or wherein the complexing agent is selected from the group comprising EDTA, complexing acids, complexing polymers, STPP, such as poly (meth) acrylic acid, and combinations thereof.

6. Method as claimed in any of the foregoing claims, wherein furthermore the amount of exchangeable Ca 2+ is determined prior to the exchange.

7. A method according to any one of the preceding claims, wherein the amount of added anion is sufficient to exchange at least 66% of the amount of exchangeable Ca 2+ present, preferably to exchange at least 90% of the amount of exchangeable Ca 2+ more preferably to exchange at least 95% of the amount of exchangeable Ca 2+, even more preferably to exchange at least 99% of the amount of exchangeable Ca 2+, such as at least 99.9% of the amount of exchangeable Ca 2+. - 22 -

A method according to any one of the preceding claims, wherein the temperature is raised to 61-75 ° C, preferably to 62-70 ° C, more preferably to 63-68 ° C.

A method according to any one of the preceding claims, wherein the temperature increase is carried out for a time of 6-720 minutes, preferably 10-240 minutes, such as 20-60 minutes.

10. A method according to any one of the preceding claims, wherein the mixture is converted by a biological conversion, such as digestion, such as anaerobic or aerobic digestion, and combinations thereof, preferably by anaerobic digestion.