WO2004035491A1

WO2004035491A1 - Procedure and device for thermophilic temperature range stabilization and mineralization of sludge from wastewater treatment plants

Info

Publication number: WO2004035491A1
Application number: PCT/SI2003/000035
Authority: WO
Inventors: Milenko Ros; Gregor Drago Zupancic
Original assignee: Kemijski Institut
Priority date: 2002-10-18
Filing date: 2003-10-06
Publication date: 2004-04-29
Also published as: SI21318A; AU2003301295A1

Abstract

The subject of this invention is thermophilic two-stage anaerobic stabilization and mineralization of waste sludge and the corresponding device. Developed process operates in thermophilic temperature range, preferably between 50 and 60 °C, and retention time of 3 to 12 days and it is divided in two stages. Because of thermophilic temperature range operation the developed process is substantially thorough and much faster. In the first- anaerobic stage the process resembles to conventional mesophilic anaerobic process. Aerobic stage after anaerobic stage completely stabilizes and removes almost all biologically degradable matter, which is not degradable anaerobically. Additional novel characteristic is also thermal regeneration. Therefore the overall heating requirements are substantially reduced and the process can be equal (from the heating requirements prospective) to a mesophilic process.

Description

Procedure and device for thermophilic temperature range stabilization and mineralization of sludge from wastewater treatment plants

Technical field of the invention

The invention belongs to the field of environmental engineering reactor technology. It relates to a new procedure of stabilization and mineralization of sludge from wastewater treatment plants, where sludge is generated as waste solids. It refers especially to an anaerobic-aerobic two-stage procedure of sludge stabilization and mineralization in thermophilic temperature range. It also preferentially includes thermal regeneration at only first - anaerobic stage of stabilization and mineralization as well as thermal regeneration at two-stage anaerobic-aerobic stabilization and mineralization. The two-stage anaerobic-aerobic procedure of sludge stabilization and mineralization is very flexible and it can be used in a wide range of needs. All capabilities of biological stabilization and mineralization of sludge can be covered with different combinations of retention times of first - anaerobic, second - aerobic or both stages.

Technical problem

At biological wastewater treatment large quantities of excess sludge are generated. This sludge. have to be additionally treated before further usβ.ore disposing. Conventional sludge treatment is anaerobic digestion in mesophilic range of temperature (30 -38°C), or aerobic digestion at temperatures of 13 °C - 20 °C. Retention time at anaerobic mesophilic digestion processes is usually between 30 and 60 days and at aerobic digestion processes between 50 to 60 days. The purpose of sludge digestion is reduction of organic solids (reduction of organic solids in conventional mesophilic process is about 40%), stabilization of microbial activity (reduction of pathogenic organisms) and at anaerobic process also biogas production. Produced biogas can be utilized with cogeneration (heat and power), to reduce the energy and heat requirements of the wastwaster treatment plant. The treated sludge can be used as fertilizer in agriculture, if the concentration of toxic or hazardous substances, for example heavy metals is not too high. In such case sludge has to be landfilled. The trend in last years (lowering the admissilble limit of heavy metals and bigger loads of heavy metals in wastewater) classifies the sludge as hazardous waste, which has to be landfilled on special sites. In Slovenia we have 126 wastewater treatment plants. 33 wastewater treatment plants have no sludge treatment, 4 treatment plants use sludge composting, the rest of treatment plants use mesophilic sludge digestion. The annual quantity of treated sludge is 35872.6 cubic meters (6608.5 tons of dry matter). 19833.6 cubic meters (3996.3 tons of dry matter) is disposed in landfills.

Current status of technology in this field

So far several two-stage processes have been developed. These processes are operating only partially in thermophilic temperature range and are designed on different concept as the process that is the matter of this invention.

Two common properties of the so far developed two-stage processes are:

• The two-stage processes are actually phase separated anaerobic processes (two phase).

• All of the processes have the first stage in thermophilic range that lasts for a relatively short time (1-3 days) and the second stage in mesophilic range that lasts much longer (12-30 days).

As known is anaerobic digestion process (from microbiological point of view) a two- phase process, composed of two phases; an acidogenic and a methanogenic phase. At this recently developed processes these two phases are divided and optimised separately. Therefore the fist stage is a thermophilic anaerobic (in some cases also aerobic) digestion, which is due to the short retention time all in the acidogenic phase. hjs _^phase, s short .(1-3 daysj^jdue to high thermal consumption of thermophilic process. Second stage is always in mesophilic temperature range; in this case it is the methanogenic phase of the anaerobic digestion process. Results achieved at such processes are published in several journals [1-6]. The main reason quoted, for not having all stages in thermophilic temperature range is that thermal requirements of the thermophilic process are very high and therefore such process is energetically not economic (especially mentioned in [1,2]).

Phase separation and separate optimisation in two-stage process contributes to better sludge degradation - higher stabilization and mineralization. Degradation of the organic sludge component is higher at shorter retention times. Biogas production is also higher than at conventional processes due to better volatile solids degradation. Biogas production is little lower at processes where the first stage is aerobic. Results of each reference are presented in table 1 : Table 1 - results of the published two-stage (two-phase) processes

The first stage of the process is in thermophilic temperature range therefore the sludge sterility is better than at conventional mesophilic processes. The number of pathogenic organisms is substantially smaller, even below infection level if the retention time is more than 48 hours [8].

References:

Tapana Cheunbarn, Krishna R. Pagilla, Aerobic thermophillic and anaerobic me^'sophillic treatment όfsfudge. Journal of environmental engineering, 2000.^"

126(9): p. 790-795.

Tapana Cheunbarn, Krishna R. Pagilla, Anaerobic Thermophillic/mesophillic dual-stage sludge treatment. Journal of environmental engineering, 2000.

126(9): p. 796-801.

3. Sambhunath Ghosh, Kevin Buoy, Larry Dressel, Terry Miller, Greg Wilcox, Dave

Loos, Pilot and full scale two-phase anaerobic digestion of municipal sludge.

Water environment and research, 1995. 67(2): p. 206-214.

Sanjoy K. Bhattacharya, Richard L. Madura, David A. Vailing, Joseph B. Farrel,

Volatile solids reduction in two phase and conventional anaerobic sludge digestion. Water research, 1996. 30(5): p. 1041-1048. 5. Juergen Oles, Norbert Dichtl, Hans-Hermann Niehoff, Full scale experience of two stage thermophillic/mesophillic sludge digestion. Water science and technology, 1997. 36(6-7): p. 449-456.

6. Yue Han, Shihwu Sung, Richard R. Dague, Temperature-phased anaerobic digestion of wastewater sludges. Water science and technology, 1997. 36(6-7): p. 367-374.

7. R. Roberts, W.J. Davies, CF. Forster, Two-stage, thermophillic-mesophillic anaerobic digestion of sewage sludge. Trans I. Chem e., 1999. 77(part B): p. 93-97.

8. Burtscher C, Fall P. A., Christ O., Wilderer P. A., Wuertz S., Detection and survival of pathogens during two-stage thermophilic/mesophilic anaerobic treatment of suspended organic waste. Water science and technology, 1998. 38(12): p. 123-126.

New solution to technical problem

The goal of the invention is to minimize the quantity of solids in sludge from wastewater treatment plants in the shortest time possible. To do so, it is required that all of the process is performed in thermophilic temperature range (50-60°C). Heat (thermal) requirements in such case are substantially higher, what was so far the main problem in process operation. Produced biogas is not sufficient for covering the heat requirements of the- process, neither for the cogeneration of heat and power~The- goal is also to completely stabilize - stop or substantially reduce any activity of the pathogenic organisms in the sludge. This can be fully achieved in the invented process with operating in thermophilic temperature range.

The claimed procedure for stabilization and mineralization of waste sludge from wastewater treatment plants in thermophilic temperature range is composed of two stages in following sequence:

• First stage - anaerobic stage, which is anaerobic stabilization-mineralization (digestion) and is performed in thermophilic temperature range from 40 to 80°C, preferably between 50 and 60°C;

• Second stage - aerobic stage, which is aerobic stabilization-mineralization (digestion) with air aeration or optionally oxygen aeration and is performed in thermophilic temperature range from 40 to 80°C, preferably between 50 and 60°C; Optionally with thermal regeneration.

One option of the process is to preheat the inflow sludge with the outflow sludge of the second stage of the process using thermal regeneration. The second option is to preheat the inflow sludge with the outflow sludge of just first stage of the process and therefore only the first anaerobic stage of the process operating.

Second subject of this invention is the device for stabilization and mineralization of sludge from wastewater treatment plants operating in thermophilic temperature range. The device includes following parts: sieve for particles (1), heat exchanger - regenerator (2), heat exchanger - heater (3), anaerobic reactor (4), optionally sludge flow pipe (5), internal combustion engine (6), electricity generator (7), aerobic reactor (8) and sludge dewatering unit (9).

The subject of invention is a new process (procedure) for stabilization and mineralization of waste sludge from wastewater treatment plants operating in thermophilic temperature range. This process is superior to the current technology in this field in following points: •—Higner-biogas production

• Higher level of volatile (organic) solids degradation in the same or shorter retention time

• Substantially lower heat requirements for the same effect.

New characteristic of the process is that it is composed of two anaerobic and aerobic processes, and not - as hitherto - of a single phase-separated process. The advantage of such two-stage process is much higher stability. An innovation is also that all stages of the process are in thermophilic temperature range and not just the first one, as it is the case so far. As mentioned in previous section the reason for having only one shorter stage in thermophilic range are high heating requirements and therefore energetically a non-economic process. In this field the invention is introducing thermal regeneration. Thermal regeneration brings the heat requirements of the process to the same level or even more favourable level as they are at mesophilic processes.

Procedure for stabilization and mineralization of sludge as the first subject of this invention is composed of two stages:

• First stage is always anaerobic due to maximum biogas production. Retention time of this stage is between 3 and 12 days. In this stage most of the organic solids are degraded, sludge COD is also reduced. Anaerobically not all of the organic components are degradable, therefore

• The second stage is aerobic. The COD is substantially reduced. Also organic solids are degraded. Retention time of this stage is also between 3 and 12 days.

Figure 1 schematically shows the device for stabilization and mineralization of sludge from wastewater treatment plants in thermophilic temperature range, which is the second subject of this invention.

• Waste sludge from wastewater treatment plants, with solids content of 30% (by mass) must first pass the particle sieve (1). All particles larger than 1 ,5 mm are excluded. This is necessary to prevent clogging of the regenerator (2), which has large surfaces for high efficiency and therefore small rifts for passing liquid, since all particles larger than 1 ,5 mm result in clogging and impairing the flow.

• - Next, waste sludge passes through the heat exchanger - regenerator (2), where- the heat is transmitted from the warm outflow sludge to the cold inflow sludge.

• Next, waste sludge is additionally heated to operating temperature (usually 55°C) by the heat exchanger - heater (3). The heat is supplied from a cogeneration module (internal combustion engine), which is fuelled by biogas produced in the first - anaerobic stage.

• Sludge than flows into the anaerobic reactor (4) - first stage of the process. Here the sludge solids are degraded by anaerobic microorganisms; solids are degraded and the sludge COD is reduced. The product of the anaerobic stabilization-mineralization is biogas, which is used to fuel the cogeneration module. Temperature of anaerobic stage is between 40 and 80°C, preferably between 50 and 60°C. • Partially stabilized - mineralised sludge flows then into aerobic reactor (8) - the second stage. Here the organic compounds in sludge are completely degraded (solids reduction is not very high, however sludge COD is drastically reduced). Temperature in the aerobic reactor is also between 50 and 60°C. Usually it is 1- 5°C lower than in anaerobic reactor due to heat losses of the aerobic reactor. Aerobic reactor must be a closed vessel (but not pressurized), to prevent water evaporation and heat losses connected to water evaporation. Therefore, aerobic reactor must be equipped with an air vent.

• After leaving the aerobic reactor (8) sludge flows again through heat exchanger - regenerator (2) to preheat the inflow sludge.

• Before further use or disposing sludge is usually dewatered (9) to reduce sludge volume.

Alternatives showed on figure 1 are also backflows (recycle), where a part of sludge is returned from reactor outflow to reactor inflow to inoculate microorganisms in the inflow.

An alternative is also, when only the first - anaerobic stage is present. In this case the retention time of anaerobic stage must be longer to assure sufficient stage of stabilization and mineralization, what also means larger reactor volume. Heat regeneration procedure is identical.' Heat' is ^"-regenerated- between "anaerobic stage - outflow and inflow. After treatment in the anaerobic reactor (4) sludge flows by pipe (5) directly to heat exchanger - regenerator (2).

All stages of the above-described process are in the above-mentioned thermophilic temperature range. This is considered advantageous compared to the processes developed so far, which only partially operate in thermophilic range. Due to operation completely in thermophilic range the process is much faster. The main reason hitherto responsible for having only one stage in thermophilic range is apparent high heating requirement necessary for sustaining process operation.

Research leading to this invention showed that heat requirements of the invented two- stage process is indeed 40-50% higher than at conventional mesophilic anaerobic processes. However, considering prior art developed two-stage processes, the heating requirements of the two-stage process of this invention is only 1 % higher.

The large deficiency of heat requirements comparing conventional and the two-stage procedure of this invention is covered by heat regeneration. Process of heat regeneration in this case is also the subject of this invention.

Thermal regeneration, which is necessary for the process of sludge stabilization and mineralization, is conducted between aerobic stage outflow and anaerobic stage inflow

(Figure 1). Heat regeneration stage of 70% or more can be achieved. Looking from heating requirements prospective, a thermophilic process using thermal regeneration can therefore be more beneficial than conventional mesophilic process.

Figure 2 shows heat balances and temperature states throughout the two-stage procedure of this invention.

• Waste sludge - sludge inflow is preheated by the heat exchanger - regenerator (2')

• Sludge is then heated to inflow temperature utilizing conventional heating cycle (70-90°C) of the cogeneration module. Here the anaerobic reactor heat losses are accounted, therefore sludge is heated little more than the reactor (4') operating temperature (in case in Figure 2 over 55°C), just enough that heat losses are covered and the temperature in the reactor is 55°C. The temperature depends- Oτr-reactσr volume.

temperatures.

• Heat losses of anaerobic reactor are minimal compared to the sludge-heating requirement (only 1-12% of overall heating requirements).

• Sludge is flowing directly from anaerobic reactor to aerobic reactor (8') with no additional heating. Temperature in aerobic reactor is therefore lower (1-5°C) than in anaerobic reactor due to the heat losses of aerobic reactor. Heat losses of aerobic reactor are covered by this sludge temperature fall.

• Heat losses, which originate in removing biogas and the aerobic stabilization and mineralization products from the reactors, are very small as well. They add up to 1% each of sludge heating requirements. • Thermal influence of aeration air to the aerobic reactor must also be considered. When designing the reactor the foreseen air temperature must be the same as in the aerobic reactor. Then this influence can be neglected.

• With aerobic sludge outflow (which has the same temperature as the aerobic reactor) the sludge inflow is preheated. Final sludge outflow temperature is between 29 °C and the temperature of the aerobic reactor, depending on the regeneration stage.

The device efficiency was determined and several times tested at National Institute of Chemistry Ljubljana, Slovenia in Laboratory for Chemistry, Biology and Technology of Water. Experiments were carried out with two reactors; anaerobic reactor volume was 20 I, aerobic reactor volume was 18.2 I.

Many combinations of anaerobic and aerobic stage retention times were determined experimentally. Retention times in each stage are between 3 and 12 days. The most important combinations are shown in Table 2:

Table 2 - experimentally determined retention time combinations of the two-stage process of this invention

We established that concerning biogas production the alternative 1 was the best, and concerning organic component removal alternatives 4 and 5 were the best.

Dimensioning of the device of this invention When dimensioning the device of this invention the following parameters are necessary:

1. Measured parameters

2. Foreseen parameters and

3. Experimentally determined parameters

Measured parameters are:

• Maxfmum daffy sTudge ffow Q_max, (m³/d)

• Minimum daily sludge flow Q_m(n, (m³/d)

• Middle daily sludge flow Q_sr, (m³/d)

• Middle COD concentration of inflow sludge COD-i (mg/l)

• Inflow sludge concentration X-i, (g/l, kg/m³)

• Organic (volatile) part of total solids in sludge (organic (volatile) component concentration of total solids) χ₀rg, (g/g, %)

• Temperature of waste sludge (inflow) f₀w. (°C).

Foreseen parameters are:

• Retention time of anaerobic stage T_an, (day)

• Retention time of aerobic stage T_ae, (day)

• Temperature of anaerobic stage t_an, (°C)

• Thickness and type of construction and insulation materials of reactors

Experimentally determined parameters are:

• Level of volatile (organic) solids removal for anaerobic and aerobic stage η₀._an, η₀-ae, (%)

• Level of COD reduction for anaerobic and aerobic stage ηcoD-an, ηcoD-ae (%)

• Specific biogas production G_bp, (l/kg of inseted volatile (organic) solids)

• Biogas composition (percent of methane in biogas) ξc_H4, (%)

Determined and dimensioned are following parameters:

• Reactor volumes (anaerobic and aerobic) V_ran, V_rae, (m³)

• Biogas flow Q_bp, (m³/day), methane flow Q_CH₄, (m³/day) • Size and type of cogeneration module

• Heat and energy capacity of the process

• Concentration of sludge solids and their volatile (organic) component in anaerobic stage outflow X_an, X₀-an, (g/l, kg/m³)

• Concentration of sludge solids and their volatile (organic) component in anaerobic stage outflow X_ae, X₀-ae, (g/l, kg/m³)

• COD of anaerobic stage outflow COD_aπ (mg/l)

• COD of aerobic stage outflow COD_ae (mg/l)

• Air flow for aeration Q_zr, (m³/h) and based on that the size of aeration devices

• Sludge heat losses and heating requirements

• Temperature of anaerobic stage sludge inflow t_0vt, (°C) and temperature of aerobic reactor t_ae, (°C)

• Necessary thermal regeneration stage from outflow to inflow sludge κ_r (%), and based on that the size and type of heat exchangers, regenerator and heater.

Reactor volume:

Anaerobic reactor: ^vran = Qn_ax ^■ P_bi ' ^Tan - Where p_bi is sludge density

Aerobic reactor:

"rae ⁼ * max ^' Pbi ^' ' ae

Biogas and methane flow:

®bp ~ r ^' Xvt ^' Zorg ^' ^b ^CH ⁼ ®bp ^' CHA

Size of the cogeneration module:

Size of the cogeneration module is determined on basis of the chemical bounded energy in methane (energy capacity). The energy capacity of the process is calculated from the lower heat capacity of methane. Qkm ⁼ QcH4 ^' HiCHA

When a certain model of the cogeneration module is selected, the producer of the module supplies the values of transformation coefficients from chemically bounded energy in methane to useful heat η_τ and electricity. The heat capacity of the process is then calculated by following equation:

Concentration of sludge solids and their volatile (organic) component in anaerobic stage outflow:

an ~ vt ' ~ Xorg ) ⁺ Xorg ^' ^lo-an ) o-an ~ ^• vt ^' Zorg ^' Vo-an

Concentration of sludge solids and their volatile (organic) component in anaerobic stage outflow: ae ⁼ vt %org ) ^"•^" ^o-an ^' Vo-ae o-ae ~ o-an ^' Vo-ae

COD of anaerobic and aerobic stage outflow:

∞Dan = T QD-an ^{' COD}vt ∞D_ae = η_C0D-ae ^' ∞D_an

Air flow (or oxygen) for aeration:

Where are:

Rkisik gas constant for oxygen

T_zr Air temperature and

K Absorption factor of oxygen absorption from air to sludge, supplied by aeration device producer K₀ Absorption factor of pure oxygen to sludge, supplied by aeration device producer (optional when aeration occurs with pure oxygen instead of K in the upper equation)

Sludge heat losses and heating requirements:

Heat losses of anaerobic and aerobic reactor:

~^~ ae ^' ae ^' V ae ^— '^■ok )

Where are: k_ae, k_an Coefficients of heat transfer for anaerobic and aerobic reactor

A_an, A_ae Surfaces of anaerobic and aerobic reactor t_0k Surrounding environment (outside) temperature

Sludge heating requirements:

Q_gr-ω = Qw ^• P_bi ^{■ c} _p-_bi ^■ (t_an - ^foω ) Where c_p._bl is specific heat of sludge

Temperature of anaerobic stage sludge inflow:

Temperature of aerobic reactor: f _ ^izg-ae

'ae ^lan ^

^ bl ^' Pbi ^{' c}p-bl

Necessary thermal regeneration stage and heat capacity of the regenerator: - ^Q9 -bl + Qizg-an - Qτ _Q _ _{Q Q} _ _Q ^{r ~} Ω ^wtø ^' P nbi ^' r -bl Λ T^ltae ~ F ^L0bAl ) ^{Tr ~ gr~bl iz9~an T}

Based on determined sludge flows, temperatures, heating requirements and regeneration stage, dimensions and type of all heat exchangers are determined. Position numbers on Figures

Figure 1

1 sieve for particles

2 heat exchanger - regenerator

3 heat exchanger - heater

4 anaerobic reactor (first stage of the process)

5 sludge flow pipe if only first stage is present

6 internal combustion engine

7 electricity generator

8 aerobic reactor (second stage of the process)

9 dewatering unit

Figure 2

2' heat exchanger - regenerator ' anaerobic reactor (first stage of the process)

8' -. aerobic reactor (second stage of the process)

Claims

Patent claims

1. Procedure for stabilization and mineralization of sludge from wastewater treatment plants in thermophilic temperature range, characterised in that it is composed of two stages in following order:

• First stage - anaerobic stage, where anaerobic stabilization-mineralization occurs in thermophilic temperature range between 40 and 80°C and retention time of 3 to 12 days;

• Second stage - aerobic stage, where aerobic stabilization-mineralization with air or pure oxygen aeration occurs in thermophilic temperature range between 40 and 80°C and retention time of 3 to 12 days; when necessary with thermal regeneration.

2. Procedure as claimed in claim 1 wherein the temperature is preferably between 50 and 60°C.

3. Procedure as claimed in claim 1 wherein sludge is preheated with thermal regeneration between second - aerobic stage outflow and sludge inflow.

4r~ Procedure -as- -claimed in claim 1- wherein sludge is- preheated - with- thermal regeneration and only first - anaerobic stage of two-stage process is operating.

5. Device for stabilization and mineralization of waste sludge from wastewater treatment plants in thermophilic temperature range, characteristised in that it is composed of: sieve for particles 1, heat exchanger - regenerator 2, heat exchanger - heater 3, anaerobic reactor 4, optionally sludge flow pipe 5, internal combustion engine

6, electricity generator 7, aerobic reactor 8 and sludge dewatering unit 9.