Optimising the biogas yield from biological sewage sludge by chemical/termic hydrolysis
The purpose is to examine methods for optimising the biogas yield of waste activated sludge. Furthemore it is the purpose of the project to evaluate the effect on sludge dewatering properties as well as a reduction of environment pollutants for specific pretreatment methods.
The main objective of the project is to improve the energy balance of sludge handling in biological wastewater treatment plants. The primary focus is on extending the use of biological return sludge (biosludge) for energy production in biological treatment plants by means of digestion tanks and shared biogas plants. This would highly ameliorate the total energy balance in municipal and industrial treatment plants. The way to achieve this is by introducing chemical pretreatment/hydrolysis with the cheap product Ca(OH)2, which will improve the relatively low biogas output from biosludge. Another alternative would be H3PO4 combined with thermal-mechanical pretreatments, which would 'open' the sludge, so to speak. Furthermore, the project aims at assessing the results of these pretreatment methods in terms of dehydration and reduction of substances injurious to the environment. An increase in the dehydration capabilities will reduce the energy consumed for transportation of sludge to the final deposit. An increased reduction in the presence of substance injurious to the environment in the sludge would mean that a larger amount of sludge would be suited for spreading on farmland, thus contributing to the intensified recycling of the vast nutritious resources, and hereby reducing the overall impact of the wastewater treatment on the environment. The project will contribute to the reduction of the overall impact of the entire wastewater treatment sector on the environment. The positive effect on the environment may partly be contributed to the improved net enegy balance for wastewater treatments, and partly to the increased reduction of the substance injurious to the environment found in the sludge, which increases the pontential for using the sludge as fertiliser. Hereby a larger percentage of the nutritious resources in the sludge will be recycled
The background for this project is that increased requirements for the treatment of waste water is causing a larger producition of waste activated sludge (WAS) at municipal wastewater treatment plants. At the same time it has become more difficult/expensive to dispose of the sludge, as the sludge must meet with certain requairements concerning content of organic pollutants as well as limit values for the content of heavy metals to be used for soil improvement purposes. Biogasification of the WAS in digesters is attractive as the process reduces the total amount of sludge and have a stabilising as well as higienic effect on the sludge. However biogasification of waste activated sludge is not an effective process compared to biogasification of primary sludge where the gas yield is approximately the double. The purpose of this report is therefore to examine methods for optimising the biogas yield of waste activated sludge. Furthemore it is the purpose of the project to evaluate the effect on sludge dewatering properties as well as a reduction of environment pollutants for specific pretreatment methods. Initially a screending of 25 different pretreatment methods has been carried out. For 15 of these pretreatment methods, there was a positive effect on the biogas yield when the yield was estimated per g VS pretreated. When the biogas yield was estimated per g VS digested, 21 of the pretreatment methods showed a positive effect. The pretreatment time for chemical methods was 24 hours, stirred under anaerobic conditions. Improvements in the biogas yield expressed in percentages was distributed as follows: 5-20% (3), 20-60% (10), 60-100% (6), and 100-115% (2). NaOH was chosen as pretreatment method for tests in CSTR model reactors, which simulate the conditions in full scale. Results showed that digestion of waste activated sludge, pretreated with 40 meq/l NaOH, provided an excess yield of methane per g VS of 63% when the waste activated sludge was pretreated at 1% dry substance (DS), whereas the improvements expressed in percentage for waste activated sludge at 3% DS was 31-40% and 18% when the waste activated sludge has a DS of 5%. However, it was concluded that pretreatment with 3% DS and a digetion of 1% DS was the method most financially advantageous. The effect on sludge dewatering was tested for the NaOH pretreated waste activated sludge by means of two test methods (CST and Fitler Belt Press). The CST test showed that the pretreatment had a negative effect on the sludge dewatering properties, but with the more realistic test, the Flexi-press, it was showed that there was not a negative effect on the dewatering properties when the sludge had been digested, in fact there was a 10% increase in the dry solid content in the dewatered sludge when pretreated
Key figures
Category
Participants
| Partner | Subsidy | Auto financing |
|---|---|---|
| Centralrenseanlægget Næstved | ||
| Søholt Renseanlæg |
Contact
P.O. Box 141
DK-2630 Taastrup, Denmark
Spendler, Finn H. (seniorrådgiver), 43504350, pia.mai@teknologisk.dk
Øvr. Partnere: Centralrenseanlægget Næstved; Søholt Renseanlæg
