Patent Description:
Sludge is typically what remains after wastewater treatment in municipal or industrial wastewater treatment plants. Municipal wastewater treatment plants treat wastewater from cities while industrial wastewater treatment plants treat water effluents from different industrial processes, for example pulp and paper mills, industrial food production facilities etc. Animal farming is also a considerable source of wastewater and sludge, for example large-scale pig farming. Embodiments of the present disclosure will be useful in all these areas.

The technologies for wastewater treatment are similar on a general level, but include specific solutions depending on the character of the waste streams to be treated, basic design, local requirements and environmental concerns. In larger plants in Sweden, the wastewater treatment process often comprises mechanical pretreatment followed by primary (settling) and secondary (aerobic) treatment steps. In some cases different forms of tertiary treatment is also applied to remove remaining problematic substances, for example drug residues, toxic organic substances etc., in the treated water. In smaller plants one or more of these stages may often be omitted.

Almost all wastewater treatment plants in use generate sludge that needs to be handled. The sludge is either recovered directly from the plant after dewatering (aerobic sludge) or first treated anaerobically for biogas production where part of the sludge is digested and the remainder is recovered as anaerobic sludge.

Wastewater treatment plants world wide produce several hundred millions metric tonnes of sludge every year and the amount is rapidly growing. In Sweden, the total sludge volume in tons of dry solids per year (tDS/y) was reported to be <NUM><NUM> in <NUM> and the current figure is estimated to be the same or higher. Sludge handling is thus an enormous challenge for society, and present solutions are associated with high cost and frequently also a negative environmental impact.

Starting from <NUM>, the European Union has adopted several directives regulating the treatment and disposal of waste water sludge, addressing different aspects such as the use of sludge as landfill, the recovery of phosphorus, incineration of sludge etc. The various directives are reflected in national legislation in the individual member states, and for example in Sweden, the disposal of sludge in landfill has been prohibited since <NUM>.

Today, the main uses for wastewater sludge are fertilization in agriculture and forestry/silviculture, mixing into plant soil for ground construction projects and the coverage and restoration of landfills, incineration with energy recovery, recovery of chemicals and the production of fertilizers, and finally landfill, however provided that the sludge has undergone specific pretreatment, such as composting.

Incineration of the sludge, with energy recovery and suitable treatment of flue gases and ashes to destroy harmful chemicals and safely handle heavy metals, remains an attractive alternative. The exact composition of the sludge however depends on the composition of the incoming wastewater and the type of wastewater treatment plant. Sludge with high concentrations of organic and/or biological components is generally difficult to dewater. The water content is frequently so high that the net heating value if incinerated in a power plant is very low or even negative and the addition of support fuels, often fossil fuel, may be necessary.

C-Green Technology AB has developed a process for treatment of sludge involving a step of hydrothermal carbonization (HTC).

The operation of many hydrothermal carbonization (HTC) systems normally requires a supply of external energy, e.g. in the form of electricity or gas. The present inventors have found that the need for continuous supply of external energy in HTC treatment of sludge can be eliminated or at least significantly reduced by wet-oxidizing the product of the HTC treatment (i.e. a HTC-treated slurry) and then subjecting the wet-oxidized slurry to flashing to obtain a high-temperature steam fraction that can be used for heating untreated sludge to the temperature of the HTC reaction. Thereby, the heat released by the oxidation reactions is recovered and used in the process in an efficient way. Another benefit of carrying out wet-oxidation downstream the HTC treatment is that the HTC treatment increases the chemical oxygen demand (COD), which means that more fuel has been made available for the wet oxidation process. Further, the HTC-treated slurry has a higher temperature than incoming or preheated sludge and higher temperature results in a higher rate of the wet oxidation reactions, which are temperature-dependent. Compared to the concept disclosed in <CIT>, the concept of the present invention is simplified, e.g. it allows for less complicated heat recovery. Further processes of HTC sludge treatment are disclosed in <CIT>, in <NPL>, and in<NPL>.

According to the invention, there is thus provided a method of hydrothermal carbonization of a sludge from a wastewater treatment plant, comprising the steps of:.

The wet oxidation of the above method results in that the COD of the liquid fraction is reduced, which decreases the load of any COD-reducing treatment (typically an anaerobic treatment) in the wastewater treatment plant. Further, the wet oxidation consumes particulate organic material, which reduces the volume of the solids fraction (that typically has to be transported to another site) and enriches the solids fraction in phosphorus and other ash components.

The oxidizing agent is preferably oxygen gas. "Oxygen gas" refers to a gas comprising at least <NUM> % oxygen by volume, preferably at least <NUM> % oxygen by volume. Consequently, "adding oxygen gas" in step b) does not cover adding air (as the oxygen content of air is only <NUM> % by volume). A benefit of using oxygen gas instead of air is that less inert gas is added to the reactor. Another benefit is a more efficient wet oxidation reaction As understood by the skilled person, the method is a continuous method.

The sludge is preferably a municipal or industrial sludge from a wastewater treatment plant.

The pre-cooled slurry may be subjected to flashing in at least two steps to obtain at least two first steam fractions of different temperatures. These at least two first steam fractions are preferably used for sequential heating of the sludge in the preheating step. In one embodiment, the pre-cooled slurry is subjected to flashing in at least three steps (e.g. three or four steps) to obtain at least three first steam fractions (e.g. three or four first steam fractions) of different temperatures, which are preferably used for sequential heating of the sludge in the preheating step.

The dry solids content (also referred to as "Total Solids") of the sludge is typically <NUM>-<NUM> %, such as <NUM>-<NUM> %, such as <NUM>-<NUM> %. If the sludge is anaerobic sludge, the dry solids content is normally <NUM>-<NUM> %. If the sludge is aerobic sludge, the dry solids content is typically <NUM>-<NUM> %. The ash content is typically <NUM>-<NUM> %, such as <NUM>-<NUM> %, such as <NUM>-<NUM> %, of the dry weight of the sludge. The higher heating value (HHV) of the sludge is typically <NUM>-<NUM> MJ/kg, such as <NUM>-<NUM> MJ/kg (dry weight).

The wet oxidation typically does not use the whole energy content of the slurry. It may for example reduce the heat content HTC-treated slurry by <NUM>-<NUM> %, preferably <NUM>-<NUM> %, more preferably <NUM>-<NUM> %. The amount of oxidation agent added in the method may be adapted accordingly. The heat content may be measured by a bomb calorimeter.

The HTC treatment has only a minor effect on the heat content of the sludge. Consequently, <NUM>-<NUM> %, such as <NUM>-<NUM> %, such as <NUM>-<NUM> % of the heat content of the untreated sludge typically remains after the wet oxidation.

The disclosed method facilitates the separation of phosphorus (P).

Accordingly, the sludge may comprise phosphorus, e.g. in an amount of <NUM>-<NUM> % of the dry weight of the sludge, such as <NUM>-<NUM> % of the dry weight of the sludge, such as <NUM>-<NUM> % of the dry weight of the sludge.

The sludge preferably comprises carbon (C), e.g. in an amount of <NUM>-<NUM> % of the dry weight of the sludge, such as <NUM>-<NUM> % of the dry weight of the sludge.

To obtain sufficient time for the wet oxidation reaction, the mixture of HTC-treated slurry and the oxidizing agent may be retained in a reactor for a period of time. The retention time in such a reactor may for example be <NUM>-<NUM>, such as <NUM>-<NUM>. The volume of such a reactor may for example be <NUM>-<NUM> %, such as <NUM>-<NUM> % of the volume of the reactor for the HTC.

The oxidizing agent may be added directly to such a reactor. In such case the reactor may be a counter- or concurrent flow reactor or an absorption tower. It may however be more preferred to add the oxidizing agent to the HTC-treated slurry upstream the reactor for the wet oxidation reactions, e.g. using a gas mixer.

When the oxidizing agent is oxygen gas, it may be added in an amount of <NUM>-<NUM> per tonne of dry sludge processed by the method, preferably <NUM>-<NUM> per tonne of dry sludge processed by the method, more preferably <NUM>-<NUM> per tonne of dry sludge processed by the method.

The average retention time in the reactor is typically <NUM>-<NUM> and preferably <NUM>-<NUM>.

The chemical oxygen demand (COD) of the HTC-treated slurry is typically at least <NUM>/l, preferably <NUM>-<NUM>/l, more preferably <NUM>-<NUM>/l.

The temperature of the HTC-treated slurry is typically <NUM>-<NUM> and preferably <NUM>-<NUM>. More preferably, it is <NUM>-<NUM>.

The wet oxidation increases the temperature of the HTC-treated slurry. The temperature of the wet-oxidized slurry may for example be <NUM>-<NUM>, preferably <NUM>-<NUM> and more preferably <NUM>-<NUM>.

The temperature of the second steam fraction, which is the high-temperature steam fraction obtained by flashing the wet-oxidized slurry, may be <NUM>-<NUM>, preferably <NUM>-<NUM> and more preferably <NUM>-<NUM>.

This second steam fraction raises the temperature of the preheated sludge. The step of further heating the preheated sludge, in which the second steam fraction is used, may for example result in a temperature increase of at least <NUM> (e.g. <NUM>-<NUM>), preferably at least <NUM> (e.g. <NUM>-<NUM>), more preferably at least <NUM> (e.g. <NUM>-<NUM>), such as at least <NUM> (e.g. <NUM>-<NUM>).

The temperature of the second steam fraction is typically <NUM>-<NUM> higher than the temperature of the heated sludge. Preferably, it is <NUM>-<NUM> higher, such as <NUM>-<NUM> higher, such as <NUM>-<NUM> higher.

Further, the temperature of the second steam fraction is typically <NUM>-<NUM> higher than the temperature of the preheated sludge. Preferably, it is <NUM>-<NUM> higher, more preferably <NUM>-<NUM> higher.

Further embodiments of the first aspect may be derived from the discussion about the second aspect below.

As a second aspect of the present disclosure, which is not part of the invention, there is provided a system for hydrothermal carbonization (HTC) of a sludge from a wastewater treatment plant, comprising:.

As understood by the skilled person, the system is adapted for a continuous process and the HTC reactor is a continuous reactor.

The oxidizing agent is preferably oxygen gas and the mixer is preferably an oxygen gas mixer. Other types of oxidizing equipment, such as counter- or concurrent flow reactors or absorption towers can also be used.

To provide time for the wet oxidation reactions, the slurry routing arrangement may comprise a second reactor arranged between the mixer and the first flashing arrangement. The mixer may also be part of a second reactor.

As less time is typically needed for the wet oxidation reactions than for the HTC process, the volume of the second reactor may be <NUM>-<NUM> %, such as <NUM>-<NUM> %, of the volume of the HTC reactor.

The second flashing arrangement may comprise at least two, such as at least three, flashing vessels arranged in series to provide steam fractions of different temperatures. Further, the preheating arrangement may comprise at least two, such as at least three, steam mixers arranged in series. The second steam routing arrangement preferably connects the flashing vessels to the steam mixers such that the sludge can be preheated stepwise.

The steam mixer(s) may be venturi mixer(s).

Otherwise, the embodiments of the invention apply to the second aspect mutatis mutandis.

<FIG> illustrate an exemplary embodiment of a system for sludge treatment according to the present disclosure, which is not part of the invention, but may be used to perform a method according to the invention.

An exemplary system <NUM> according to the present disclosure is schematically illustrated in <FIG>. A sludge is received from a wastewater treatment plant <NUM>. The sludge may be received directly from the plant <NUM> or from a storage tank (not shown) that forms part of the system <NUM>. The sludge typically has an initial temperature of about <NUM> and a dry matter content of about <NUM> %. After optional initial heating (not shown), the sludge is preheated in a preheating arrangement <NUM>. The preheating is preferably carried out by stepwise additions of steam, e.g. in a first <NUM> and a second <NUM> steam mixer arranged in series. Downstream each steam mixer <NUM>, <NUM>, a pump 104b, 105b is arranged. After the preheating arrangement <NUM>, a preheated sludge having a temperature of about <NUM> is obtained. The preheated sludge is heated further in a further heating arrangement <NUM>, which is typically a steam mixer and after which a pump 106b is arranged.

The further heated sludge is fed to a reactor <NUM>, in which the sludge undergoes hydrothermal carbonization (HTC). An HTC-treated slurry, which typically has a temperature of <NUM>-<NUM> is withdrawn from the reactor <NUM>. The pressure of the HTC-treated slurry is slightly increased a pump (not shown). Oxygen gas is then added to the HTC-treated slurry in an oxygen gas mixer <NUM>. The oxygen gas mixer is connected to a pressurized oxygen tank (not shown). The amount of oxygen gas may be about <NUM> per tonne of dry sludge processed in the system <NUM>. The wet oxidation is not instantaneous. Rather, it will be ongoing when the fraction flows downstream the oxygen gas mixer <NUM>. To allow time for the exothermic wet oxidation reactions, a second reactor <NUM> may therefore be arranged downstream the oxygen gas mixer <NUM>. The wet-oxidized slurry obtained from the second reactor <NUM> typically has a temperature of about <NUM> and is led to a first flashing arrangement <NUM> producing a high-temperature steam fraction having a temperature of about <NUM> and a pre-cooled slurry. The high-temperature steam fraction is routed to the further heating arrangement <NUM> and is thus used to heat the preheated sludge.

The pre-cooled slurry is subjected to flashing in a second flashing arrangement <NUM>, which produces at least one steam fraction that is used to preheat the sludge in the preheating arrangement <NUM>. Preferably, the flashing arrangement <NUM> comprises several flashing vessels arranged in series to produce steam fractions of different temperatures. For example, the flashing arrangement <NUM> may comprise: a first flashing vessel <NUM> that produces a steam fraction of "medium" temperature that is routed to the second steam mixer <NUM> of the preheating arrangement <NUM>; and a second flashing vessel <NUM> that produces a steam fraction of relatively low temperature that is routed to the first steam mixer <NUM> of the preheating arrangement <NUM>.

The cooled slurry obtained downstream the second flashing arrangement <NUM> is separated by a separating arrangement <NUM> into a liquid stream that is routed back to the wastewater treatment plant <NUM> for further treatment and a solids fraction that has been enriched in ash components, such as phosphorous, by the upstream wet oxidation process.

The system <NUM> may comprise a heater <NUM> using external heat, such as an electrical heater, for cold-starting the process. The heater <NUM> is preferably arranged downstream the further heating arrangement <NUM>, but upstream the reactor <NUM>.

The preheating arrangement <NUM>, the further heating arrangement <NUM>, the associated pumps and the heater <NUM> forms part of a sludge routing arrangement <NUM> for routing sludge to the reactor <NUM>.

Claim 1:
A method of hydrothermal carbonization of a sludge from a wastewater treatment plant, comprising the steps of:
preheating the sludge with at least one first steam fraction to obtain a preheated sludge;
further heating the preheated sludge with a second steam fraction to obtain a heated sludge;
subjecting the heated sludge to hydrothermal carbonization (HTC) in a reactor to obtain a HTC-treated slurry;
mixing the HTC-treated slurry with an oxidizing agent, such as oxygen gas, to obtain a wet-oxidized slurry;
subjecting the wet-oxidized slurry to flashing to obtain the second steam fraction and a pre-cooled slurry;
subjecting the pre-cooled slurry to flashing in at least one step to obtain the at least one first steam fraction and a cooled slurry;
separating the cooled slurry into a liquid fraction and a solids fraction; and
routing the liquid fraction to the wastewater treatment plant for further treatment,
wherein the second steam fraction is used for heating preheated sludge to the temperature of the HTC reaction.