Operating experiences with thermal sludge disintegration at the Lingen/Ems wastewater treatment plant

Transcription

1 Reprint of KA Korrespondenz Abwasser, Abfall 63rd year, issue 3/2016, page Operating experiences with thermal sludge disintegration at the Lingen/Ems wastewater treatment plant Marianne Buchmüller (Grafenhausen), Ulrich Knörle (Ravensburg) and Laurenz Hüer (Lingen/Ems) Abstract Operating Experiences with Thermal Sludge Disintegration in the Lingen/Ems A new process for the thermal disintegration of sludge is being tested in the Lingen/Ems wastewater treatment plant, within the scope of a project sponsored by the German Federal Ministry for Environment, Nature Conservation, Building and Nuclear Safety (BMU). This is to contribute to the increase of the energy efficiency of the whole plant. After three years running of the project it appears that, through the thermal disintegration, the gas production is increased, the organic substance better degraded and the sludge is more easily dewatered with equilaterally reduced application of polymers. The results show that the integration of the thermal sludge disintegration in the operation of the wastewater treatment plant, and also with wastewater treatment works of medium size, can bring significant benefits and savings. Key words: sewage sludge, treatment, disintegration, thermal, dewatering, heat generation, energy generation, gas, combined heat and power plant, operating experience 1 Introduction Wastewater treatment plants are usually considered the biggest individual consumer in the municipal energy balance. However, the wastewater processing plants also have potential for production of energy. The example of the wastewater treatment plant of the Lingen urban drainage in the context of the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMUB) s large scale Energy efficient wastewater treatment plants project with funding from the environment innovation programme, Lingen: Plus-energy wastewater treatment plant with phosphor recovery will show that existing wastewater treatment plants can be converted not only to zero-energy plants but actually into energy generators with a positive energy balance. A key element in this is thermal disintegration of the surplus sludge. It is also planned to recover 30 % of the phosphor from the sludge flow in relation to the total freight contained in the wastewater treatment plant inflow. This contribution is predicated on additional large scale pilot studies carried out outside the funding project and initial operating experiences with thermal disintegration in this three-year pilot study. 2 Point of departure The Lingen in Emsland urban drainage facility collects and cleans wastewater from the city and from a neighbouring municipality. Each day the system treats around 14,000 cubic meters of wastewater. The wastewater treatment plant occupies 25 employees and has an expansion capacity of PE. The actual load in relation to the COD freight however is only around 140,000 PE (2011). Of this, approx PE are municipal wastewater and PE come from industrial enterprises. The biggest industrial discharger is a manufacturer of acrylic fibres whose wastewater contains a high proportion of COD which is slowly degradable. The high percentage of industrial wastewater of 55 % means that the COD concentration in the inflow is somewhat higher than usual. The average proportion of primary and surplus sludge is on the contrary less than the value from a recalculation of the wastewater treatment plant according to DWA-A 131 [1]. To increase the digester gas yield, before the implementation of thermal disintegration, approx. 3,000 m³/a wastewater with a high COD content and highly degradable from biodiesel production were treated as co-substrate in the two digesting tanks. Figure 1 shows a simplified block diagram of the wastewater treatment plant before the start of the project. The Lingen wastewater treatment plant employs two (Bucher) slurry filter presses for digested sludge dewatering. The dewatering operating data in the original system clearly showed that relatively high specific polymer use of approx. 19 kg AS/Mg DM plus additional conditioning with approx. 230 kg/mg DM ferrous solution was required to achieve acceptable solid concentrations of approx. 26 % DM in the sludge cake. en.dwa.de a Korrespondenz Abwasser, Abfall 2016 (63) No. 3

2 2 Reprint of KA Korrespondenz Abwasser, Abfall 63rd year, issue 3/2016, page Fig. 2: Design of the thermal disintegration system (1 Feed pump, 2 Preheater, 3 Tubular reactor, 4 Disintegration reactor, 5 Cooling stage, 6 Thermal oil circuit, 7 Regenerative system) Fig. 1: Simplified illustration of the Lingen wastewater treatment plant 3 Process The project in Lingen entailed the use of the LysoTherm process, a patented system for the thermal disintegration of organic sludges. The aim of this process: improvement of the anaerobic stabilisation (digestion) of organic sludges. This means to achieve: increase of gas yield reduction of the dry material part in digested sludge improvement of sludge dewatering increase of digestion tower capacity. Other benefits of thermal sludge disintegration are reduced sludge viscosity, reduced foaming tendency in the digestion tower, increase in phosphor recovery potential from the sludge flow and the elimination of pathogenic bacteria in sludge [2]. Figure 2 shows the design principle of the system. The sludge passes through the sludge pump (feed pump, 1) into a multi-stage heat exchanger system. It is pumped in continuously. Preheating takes place in the first stage of the heat exchanger system (2). In the next stage the tubular reactor (3) heats the sludge to the selected reaction temperature. The actual thermal disintegration takes place at the specified reaction temperature without further heat input into the disintegration reactor (4). The sludge generally remains there for 30 to 60 minutes. After disintegration the cooling stage (5) cools the sludge until it reaches the required temperature for entry into the digestion tower. Alternatively it can be mixed with cold primary sludge at digestion chamber temperature. The system heating is carried out by two heating circuits: The thermal oil circuit (6), which provides the necessary process heat in the tubular reactor. This is recovered from the exhaust gas heat of the combined heat and power units. This allows use of a source of heat whose energy is otherwise lost directly into the atmosphere. Fig. 3: Heat extraction from the combined heat and power units with heating water and thermal oil circuits The regenerative circuit (7), in which water is used as the heat carrier. This system recovers heat from the thermally disintegrated sludge, which is then used to provide the heat for preheating. As well as a high degree of heat recovery, the development of the process also took into consideration optimum heat insulation. Thus the exhaust gas heat recovered by the CHP unit is sufficient to operate the plant and no digester gas is used to generate the process heat. Figure 3 is a diagram showing the heat extraction from the CHPs. The heat is extracted from the CHP at two different stages. firstly from the motor cooling circuit with heating water at a typical temperature of approx. 84 C and secondly from the exhaust gas via thermal oil at a temperature of approx. 180 C for the operation of the thermal disintegration system. The heating water reaches an buffer tank with a capacity of approx. 15 m³, which is also the interface for feeding into with the district heating network. If the temperature in the buffer tank return exceeds 70 C, excess heat is discharged via the emergency cooler to the atmosphere so that the return tem- a Korrespondenz Abwasser, Abfall 2016 (63) No. 3 en.dwa.de

3 Reprint of KA Korrespondenz Abwasser, Abfall 63rd year, issue 3/2016, page perature does not exceed the specified maximum value and even if there is too little heat uptake, the CHP can operate at any time. The exhaust gas from the CHPs passes through an exhaust gas heat exchanger where it is cooled from originally 500 C to approx. 180 C. The exhaust gas cooling is deactivated at a specific level to prevent condensate from forming in the exhaust gas section. With an electrical capacity of 300 kw approx. 130 kw high temperature heat can be recovered. If less or no heat is needed in the thermal disintegrator, excess heat can be transferred to the heating water inflow via a thermal oil and water heat exchanger. The high temperature heat is thus converted to low temperature heat via this heat exchanger. This design means that the temperature in the hot water inflow to the accumulator can rise to 95 C. Here too it is ensured that the CHP operation is de-coupled from dependent systems. In future at Lingen, the thermally disintegrated surplus sludge will be digested separately from the primary sludge. This process is called LysoGest and amongst other things it enables a greater concentration of nutrients in this case particularly phosphor and improves its recovery as Magnesium Ammonium Phosphate (MAP). The separate digestion of primary sludge and thermally disintegrated surplus sludge means a far greater content of ortho phosphate in the surplus sludge digestion tower, which on the one hand increases the potential for phosphate recovery and at the same time aids MAP deposition in the digestion tower. In the primary sludge digestion tower, the lack of iron from the phosphate precipitation can lead to an increase in the hydrogen sulphide concentration in the digester gas. Both effects are the object of further investigations and will affect the decision over the future running method of the digestion towers. Fig. 4: Original configuration of the wastewater treatment plant at Lingen 4 Integration Figures 4 and 5 show the original sludge line and the phased integration of thermal disintegration and separate sludge digestion into the existing wastewater treatment plant at Lingen. In the original sludge line system the primary sludge flowed firstly through a thickening in the primary sedimentation, and then in parallel to the two digestion towers. The surplus sludge was thickened via a belt thickener with the aid of polymers to a dried matter content of approx. five percent and was also fed into the two digestion towers. The primary sludge was deducted from the primary sedimentation, synchronised in time over a 24 hour cycle. The mechanical surplus sludge belt thickener was also in constant operation and was taken out of operation for maintenance and cleaning purposes only. The digestion towers have separate circulation circuits for heating and mixing (not shown). Each digestion tower has its own circulation circuit; the heating circuit however switches periodically between the two digestion towers. Due to spatial conditions, the primary sludge and the thickened surplus sludge were fed into the heating circuit of the digestion towers. The co-substrate was also fed to the digestion towers via the heating circuit. At the end of the process the two Bucher presses dewatered the digested sludge. Due to the high capacity of the dewatering units, operation was required for a maximum four to five days a week. The digested sludge is temporarily stored in a storage facility with a capacity of approx. 550 m³. After a brief holding Fig. 5: Step-by-step implementation of thermal disintegration and separate digestion period the dewatered sludge is taken by truck to a silo for combustion in a coal-fired power station. In the first project phase the thermal disintegration was temporarily switched to the dewatered surplus sludge run-in line to the digestion tower. Parallel loading of the digestion tanks continued. The raw sludge and the co-substrate were evenly distributed to both digestion tanks. Since the additional implementation of LysoGest in the second project phase, the two digestion tanks can now be loaded totally independently. Primary sludge and co-substrate now enter digestion tower I and the thickened and thermally disintegrated surplus sludge are fed to digestion tower II. The heat recovery of the disintegration plant is designed to provide all the necessary heat for the digestion tower II in the winter half en.dwa.de a Korrespondenz Abwasser, Abfall 2016 (63) No. 3

4 4 Reprint of KA Korrespondenz Abwasser, Abfall 63rd year, issue 3/2016, page year. In the summer half year, the temperature is maintained at an optimum level by a temperature regulator. Thermal oil heat exchangers in the exhaust gas flow of the two CHP units generate the process heat for the operation of the disintegration plant. Since there is no need for simultaneous hot water heating of digestion tower II, the quantity of hot water needed to operate the system and the digestion is almost identical to that of the original system. The only difference is the higher temperature of the thermal oil heat exchanger (approx. 170 C) as compared to the heating water (approx. 80 C). This is particularly important because the heating energy generated by the CHPs and not used in the wastewater treatment plant is fed into a municipal district heating network. At the same time this is an added reason to maximise the digester gas yield. 5 Pilot tests It was expected that the dewatering results achieved with the slurry filter presses in Lingen to other wastewater treatment plants would have limited transferability, so large scale technical trials were carried out with dewatering centrifuges (Figure 6). Centrisys and Hiller units were used for the trial, with operating parameters set to standard values for large scale operations. In both cases the dewatering trials ran over a period of several weeks. Since the recovery of phosphor also makes dewatering of the digestion sludge easier, it made sense for it take place before the dewatering of the sludge flow. Therefore a MAP precipitation pilot plant was operated for several months to investigate particularly the effects of the slurry filter presses and the centrifuge on the operating results. 6 Results 6.1 Phase 1: Implementation of thermal disintegration Table 1 shows the in- and outflow values of the digestion tank in 2011 (reference status) and 2013 (with disintegration, Fig. 6: Dewatering trials after phase 1 Phase 1). It is assumed that the reduction of the DM (dry matter) and odm (organic dry matter) freights is due to the following factors: Reduction of COD freight in the intake of the biological stage of the wastewater treatment plant from 3901 t/a in 2011 to 3776 t/a in 2012 Decommissioning of an activated sludge tank in May 2011 Increased proportion of biological phosphate removal within the overall phosphate removal (biological chemical) from 48.9 % to 59.8 %. As it is to be assumed that the co-substrate is fully degraded in the digestion, its influence in this balance of DM and odm freights remains the same. While the disintegration plant only went into operation in March 2012 and the first few months were used to make adjustments to the operating parameters, there is already a significant increase in odm decomposition in digestion from 43 % originally to 47 % now. The increased degree of degradation is entirely due to the increased degradation of surplus sludge as a consequence of the thermal disintegration. Laboratory tests first determined the degree of degradation of primary sludge and surplus sludge. Based on these results and the actual quan- Parameter DM odm DM odm kg/m³ t/a % t/a kg/m³ t/a % t/a DT inflow PS % % 1084 SS % % 983 Sum % % 2067 Comparison to % 89.9 % DT outflow DS % % 1089 Comparison to % 83.1 % Degradation of DM and odm (in relation to overall raw sludge) 33 % 43 % 35 % 47 % Table 1: Measured values before and after the implementation of thermal disintegration a Korrespondenz Abwasser, Abfall 2016 (63) No. 3 en.dwa.de

5 Reprint of KA Korrespondenz Abwasser, Abfall 63rd year, issue 3/2016, page tity ratios we can calculate that the odm degradation of the surplus sludge rose from 25 % originally to 33 %. This corresponds to a relative increase of around one third. 6.2 Dewatering trial after phase 1 After the conclusion of phase 1, large scale dewatering of the sludge to approx % DM was carried out with the help of the locally installed slurry filter presses. Compared to the original results this represents an increase of approx. three percent. These results were exceeded significantly with the centrifuge, in which extraction rates were up to 30 % DM. Polymer consumption on the slurry filter press rose during phase 1 by 3 to 5 kg AS/Mg DM to approx. 22 to 24 kg AS/Mg DM, this was significantly reduced with the use of the centrifuge to around 14 kg AS/Mg DM. The thermally disintegrated, digested and dewatered digested sludge is very crumbly in texture which implies that subsequent drying will be significantly easier (Figure 7). 6.3 Phase 2: Implementation of disintegration and separate sludge digestion Table 2 shows the in- and outflow values of the digestion tank in 2011 (reference status) and 2014 (disintegration separate sludge digestion, phase 2). Compared with the reference year 2011, a further reduction of the DM and odm freight for digestion was recorded in This is caused by a further reduction from 5798 Mg/a to 4669 Mg/a of COD entering the wastewater treatment plant. The proportion of biological phosphate elimination stabilised in the investigation period to the value determined in 2012 of approx. 59 %. The data shows that, again in comparison to the year 2012 (Phase 1), there was a further increase in the degradation of the odm of the total sludge volume (primary and surplus sludge) to 54 %. The odm degradation of the surplus sludge was 51 %, which corresponds to a relative increase of more than 100 % compared to the original status. With the increased degradation of the organic substance, increased gas production was also observed. In digesting tower 2 (disintegrated surplus sludge) this rose from originally 0.25 m³/kg odm feed to approx m³/kg odm feed. The relative increase is approx. 92 % and Fig. 7: Crumbly structure of thermally disintegrated, digested and centrifugally dewatered digested sludge is thus somewhat below the relative increase of the odm degradation. Laboratory tests have shown that more than 90 % of the digestion gas recovered from disintegrated surplus sludge is formed after just a few days. This behaviour was also clearly observable during the large scale pilot studies in the quick reaction of the gas production to fluctuating inflow rates. This showed that even on a large scale, the digesting capacity can be significantly increased with upstream surplus sludge disintegration. 6.4 Dewatering trial after phase 2 Dewatering the digested sludge via the slurry filter presses involved high consumption of conditioning agents (polymer, ferrous solution), even during phase 2, and the consumption fluctuated significantly. Probable causes for this were the extreme variations in composition of digested sludge in the sludge tank between the digestion towers and the dewatering units. Pretests indicated that the dewatering behaviour is heavily dependent on the quantity ratio of the various digestion sludges from digestion tower 1 (primary sludge and co-substrate) and digestion tower 2 (thermally disintegrated surplus sludge). At the same time there was also a noticeably highly fluctuating polymer consumption in the parallel dewatering via a Parameter DM odm DM odm kg/m³ t/a % t/a kg/m³ t/a % t/a DT inflow PS % % 1058 SS % % 933 Sum % % 1992 Comparison to % 86.6 % DT outflow FS % % 925 Comparison to % 70.6 % Degradation of DM and odm (in relation to overall raw sludge) 33 % 43 % 43 % 54 % Table 2: Measured values before and after the implementation of the thermal disintegration and separate sludge digestion en.dwa.de a Korrespondenz Abwasser, Abfall 2016 (63) No. 3

6 6 Reprint of KA Korrespondenz Abwasser, Abfall 63rd year, issue 3/2016, page Sludge DM content Polymer consumption Separation rate Digested mixed sludge (PS SS) with Fe conditioning (4 L/m³) without MAP precipitation Mixed sludge (PS SS) without Fe conditioning with MAP precipitation Digested and hydrolysed SS with Fe conditioning (6 L/m³) without MAP precipitation Digested and hydrolysed SS without Fe conditioning with MAP precipitation Digested and hydrolysed SS with Fe conditioning (4 L/m³) with MAP precipitation Digested PS with Fe conditioning (1.2 L/m³) without MAP precipitation Digested PS with Fe conditioning ( L/m³) without MAP precipitation Dewatering via Bucher Press % kg/mg DM. 99 % 34 % kg/mg DM. 98 % % kg/mg DM. 98 % % kg/mg DM. 96 % % kg/mg DM. 99 % 32 % 8 kg/mg DM. 99 % % kg/mg DM. 99 % Table 3: Compiled test results portable centrifuge, although the polymer consumption here was far less than with the Bucher presses. One of the causes of high polymer consumption with the Bucher presses is presumably the comparatively low DM content of the digested sludge of just around two percent. The trial program with the centrifuge entailed recording the data for the following sludges after digestion: Mixed sludge Quantity-proportional mix from the outflows of digestion tower 1 (primary sludge digestion) and digestion tower 2 (digestion of disintegrated surplus sludge) Thermally disintegrated surplus sludge Outflow from digestion tower 2 Primary sludge Outflow from digestion tower 1. Table 3 shows the trial results achieved with the centrifuge tests along with details of sludge conditioning. It also contains results from the dewatering of digested primary sludge on the slurry filter presses. The test results clearly show the positive effect of thermal disintegration on dewatering. Compared to the original system in which an average of approx. 26 % DM was achieved with a polymer consumption of approx. 19 kg/mg DM, a significantly higher dry substance content in dewatered sludges was found in all thermally disintegrated sludges over the investigation period. Polymer requirement was also found to be reduced, at times considerably. Dewatering of the digested primary sludge via the slurry filter presses in the trial period was found to produce a very high dry substance content in the dewatered sludge and an acceptable polymer consumption. Conditioning with iron had a positive effect on the separation rate over the investigation period. Additional MAP precipitation in the sludge flow before dewatering was also found to have positive effects on the dewatering rate ( 4 to 5 percent in the case of DM) and on polymer use. The results shown will be verified after the end of the project in a one-year measurement phase. 7 Operating experiences The experiences to date in the investigation period show that the plant personnel found operation of the disintegration plant safe and problem free at all times. The time taken for mere plant operation was estimated at approx. one hour a day. In addition, mandatory analytical measurements were carried out in relation to the funding project. Thanks to the largely automated operating method, extremely minimal manual intervention was required of operating personnel. Operation of the thermal disintegrator required approx. 160 kwh/d and of the thermal oil system approx. 200 kwh/d (average values June 2015) of electrical energy. This meant that only around one third of the additional gas yield of approx. 500 m³/d was used by the CHPs for the provision of electrical energy. All the high temperature heat required to operate the thermal disintegrator can be recovered from the CHPs exhaust gas heat exchangers. Therefore the high temperature heat power used was only approx. 80 to 90 kw (June 2015). Even with the operation of just one CHP at minimum load a Korrespondenz Abwasser, Abfall 2016 (63) No. 3 en.dwa.de

7 Reprint of KA Korrespondenz Abwasser, Abfall 7 (60 % of the maximum output 180 kw el ) the high-temperature heat produced is sufficient to guarantee the operation of the disintegration plant. The additional gas yield associated with the increased odm decomposition is therefore also available for increased production of electrical energy and need not be used for direct generation of thermal energy or removed from bivalent use. 8 Cost / benefit Table 4 shows the costs and benefits of thermal disintegration. Accordingly the investment costs of are compared to operating costs of /a and revenues of Therefore an amount of /a is generated from the operation of the thermal disintegration system. 9 Conclusion Particularly in Phase 2 of the investigations the effect of thermal disintegration was very obvious. The increased gas production, the improved odm degradation of the disintegrated sludge and the increased dewatering rate with coinciding reduced polymer consumption are all commercial benefits of the process. The non-use of steam as an energy carrier and the use of waste gas heat in the CHP along with a high level of heat recovery mean that the added gas yield also contributes to a higher in-house generation of electrical energy. There is no direct combustion of digester gas required to generate the necessary process heat. Investment costs LysoTherm, including thermal oil system 1,350,000 Total investment costs 1,350,000 Operating costs Electricity /a 19,600 Maintenance /a 15,000 Personnel /a 14,000 Chemicals /a 3000 Total operating costs /a 51,600 Revenues Increased in-house electricity generation /a 175,000 Increased heat generation (sold to district network) /a 45,000 Reduction of dewatered sewage sludge freight /a 100,000 Reduced chemical consumption /a 15,000 Total revenue /a 335,000 Table 4: Investment and operating costs and revenues from the use of thermal disintegration (all values net, i.e. plus VAT) The investigation results show that the implementation of thermal disintegration in the wastewater treatment plant operation offers considerable savings, including for medium-sized wastewater treatment plants. The combination of thermal disintegration with the switch to separate digestion further increases the degradation rate and again significantly improves de-watering of the digested sludge, particularly in combination with MAP precipitation from the sludge flow. The increased capacity of the existing digestion, achieved by implementation of disintegration, also provides additional freedom to accept external sludges or co-substrates with which gas production can be further increased. With respect to the increased revenue of around 278,000 /a after deducting operating costs, operation of the thermal disintegration system offers clear commercial benefits for the wastewater treatment plant. The investment costs relate to the installation in the Lingen wastewater treatment plant. Optimisation of the production process has greatly reduced investment costs for these types of plant, with expectations of even greater commercial potential of new systems. After the end of the project, including the measurement program and success monitoring, all relevant results will be published in a final project report. The final report is expected to be ready by mid It will then be available on the Internet at: Literature [1] ATV-DVWK-A 131: Bemessung von einstufigen Belebungsanlagen, Hennef, 2000 [2] DWA, Merkblatt DWA-M 302 Klärschlammdesintegration (Gelbdruck), DWA, Hennef, [3] Stadtentwässerung Lingen, Förderantrag KfW BMU-Umweltinnovationsprogramm, Authors Dr. Marianne Buchmüller Eliquo Stulz GmbH Beim Signauer Schachen 7, Grafenhausen marianne.buchmueller@eliquostulz.com Ulrich Knörle Eliquo Stulz GmbH Jahnstraße 36, Ravensburg ulrich.knoerle@eliquostulz.com Laurenz Hüer Stadtentwässerung Lingen Waldstraße 31, Lingen (Ems) Hueer@stadtentwaesserung-lingen.de A en.dwa.de a Korrespondenz Abwasser, Abfall 2016 (63) No. 3

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