Anaerobic process. An introduction. Ecochemicals UAB

Transcription

1 Anaerobic process An introduction Ecochemicals UAB

2 When anaerobic process is used? Anaerobic process is usually used for: effluents that are highly polluted with organics, usually COD (BOD) varying from 2000 to mg/l For biological and mixed (primary with biological) sludge digesting

3 What anaerobic process can do that aerobic can t? Anaerobic process permits to lower highly concentrated organic contents with low energy cost or even with positive energy output This happens because anaerobic process (differently than aerobic) removes organics without aeration, that requires a high amount of energy Despite that, anaerobic process yields methane gas, that is valuable energy source

4 What anaerobic process can t do that aerobic can? Organic content treated in anaerobic process retains relatively high values after treatment o After anaerobic treatment BOD of effluent always starts from few hundreds mg/l and higher o This means that effluent after anaerobic treatment can never be discharged to natural waters o It usually passes additional aerobic treatment in local or in municipal waste water treatment plant through sewerage system o Modern aerobic process also permits to decrease nitrogen and phosphorus levels till tenths or even few mg/l

5 Main differences between the processes are as follows: Anaerobic process provides possibility to decrease BOD from thousands to hundreds mg/l Aerobic process permits decrease from hundreds to tenths of mg/l Aerobic process uses energy and anaerobic ñ yields it BOD,

6 Main types of anaerobic process according temperature Mesophilic process ñ Temperature inside methane tank is between o C Thermophilic process ñ Temperature inside methane tank is between 50 ñ 60 o C Thermophilic process is faster that mesophilic, thus requires less volume of methane tank for the same amount of pollution On other, hand thermophilic process is less resistant to changes of pollution or volumes of incoming material Thermophilic process needs extra heating Thermophilic process produce sludge with worse dewaterabilty characteristics and higher smell There is a strict distinction between mesophilic and thermophilic processes. They both can never take place in one methane tank. Because mesophilic microorganisms are killed very quickly in temperatures starting from 45 o C. And thermophilic microorganisms are inactive at lower temperatures

7 Main types of anaerobic process according to filling manner Continuous. o ñ Anaerobic tank is continuously filled, and excess of sludge and effluents is continuously discharged. o ñ Can work for years without breakdown o ñ In order to ensure stable process incoming substance has to keep stable flow rates, type and amount of pollution Batch or periodic o ñ Anaerobic tank is filled at once and discharged only after process is finished o ñ More stable process o ñ Requires larger amount of anaerobic digesters for keeping uninterrupted process, thus is more expensive

8 Classification according of dry solids content in incoming substance I. Anaerobic process can be classified according the incoming material that is going to be treated. Dry solids content of incoming substance defines construction of reactors, speed of process and etc. All listed below types of anaerobic processes can be mesophilic or thermophilic

9 Classification according of dry solids content in incoming substance II. Relatively the anaerobic processes according to incoming substance we can divide to: Anaerobic treatment of effluents ñ low dry solids content in incoming material (from 0 to mg/l i.e. from 0 till 0,5%) o ñ Industrial effluent with BOD from thousand till tenths of thousands mg/l. (From diary, spirit, bio-diesel, sugar, paper end etc. industries) Sludge digestion ñ high dry solids content in incoming material (from to mg/l i.e. from 0,5 till 40 %) ñ municipal or industrial sludge from WWTP o ñ manure o ñ green vegetable matter o ñ By-product wastes from abattoirs & food processing o ñ Fruit cannery wastes o ñ Flour mill wastes and etc.

10 Classification of anaerobic effluent treatment processes Anaerobic suspended growth treatment process ñ Anaerobic contact process ñ Up-flow anaerobic sludge blanket process Anaerobic attached growth process ñ Anaerobic filter solid media process ñ Expanded bed process (i.e.sand, coal as media) Combined ñ Multi stage processes

11 Classification of anaerobic sludge digestion processes Anaerobic suspended growth treatment process o ñ Anaerobic sludge digestion Conventional standard rate single stage process digester content is unmixed and stratifying into layers) High rate-complete mix single stage process (digester content is completely mixed and non stratified) Two stage processes (typically the combination of high rate process in first digester and conventional in the second.)

12 Distribution of anaerobic processes in Europe Mesophilic types are prevailing, though thermophilic segment is increasing 90 % are single stage, only 10 % are multi stage processes Most popular process for effluent treatment: ñ Mesophilic, suspended growth, single stage process: Continuous Completely mixed Contact Most popular process for sludge digestion: ñ Mesophilic, suspended growth, single stage process: Incoming sludge with less than 10 % of dry solids Continuous High rate - completely mixed

13 Scheme of typical effluent treatment methane tank Contact, suspended growth, single stage, continuous, completely mixed effluent treatment methane tank:

14 Scheme of typical sludge digester Suspended growth, single stage, continuous, high rate - completely mixed sludge digester:

15 Main parameters for typical effluent treatment methane tanks* Mesophilic, continuous, suspended growth, single stage, contact process for effluent treatment Load of effluents is expressed in BOD or COD mg/l, usually have from mg/l Biomass concentration inside methane tank is 2 ñ 5 % Load is 0,5 ñ 2,5 kg of COD**/m 3 of methane tank per day Operation temperature 30 ñ 38 o C Hydraulic retention time (time that effluent is placed in the tank) 2 ñ 10 hours Removal of COD and conversion it to biogas in the tank is % 1 kg of COD fed to methane tank produce 0,25 ñ 0,4 m3 of biogas*** 1 kg of COD destroyed in methane tank yield 0,3 ñ 0,5 m3 biogas*** ph inside methane tank ñ 6,6-7,6 *These data are approximation and for general impression about the process only. **COD is always higher than BOD. Difference can be from 1,2 to 2 or 3, or even more times. *** The volume is valid under normal conditions, when press. is 1 atm, and t is between 0 and 20 o C

16 Main parameters for typical sludge digesters * Mesophilic, continuous, suspended growth, single stage, high rate ñ completely mixed sludge digesters Load (incoming sludge) is expressed in of dry (or volatile) solids (DS or VS), usually has concentration less than 10 % Load is 2-6 kg of VS/m 3 of methane tank per day Operation temperature 30 ñ 38 o C Hydraulic retention time (time that sludge placed in the tank)15-35 days. Though process is continuous we can assume that this shows average time that each particle of incoming sludge will spent in methane tank. Conversion of VS to biogas in the tank is 35-45% during days. 1 kg of VS fed to methane tank produce 0,25 ñ 0,5 m3 of biogas** 1 kg of VS destroyed in methane tank yield 0,5 ñ 1m3 biogas** ph inside methane tank ñ 6,6-7,6. *These data are approximation and for general impression about the process only. **This volume is valid under normal conditions, when pressure is 1 atm, and t is between 0 and 20 o C

17 Biogas Biogas from all types of methane tanks normally consist of 65-75% methane (CH 4 ), and 25 ñ 30 % of carbon dioxide (CO 2 ) The rest is free nitrogen (N 2 ), hydrogen (H 2 ); Sulphur hydrogen (H 2 S), water (H 2 O) vapor 1m3 of biogas at normal conditions (1 atm; 0 o C) will have around kj energy and produce around 2 kwh electrical energy (At 35% typical conversion efficiency of thermal to electrical energy)

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21 Questionnaire for evaluating of methane process Is costumer satisfied with anaerobic process? ñ If no, what main problems on his opinion? Thermophilic or Mesophilic process? What temperature in the methane tank? What is ph in the methane tank? What is incoming material? ñ ñ If effluent: what BOD or COD? How they separate active sludge from treated effluent? If sludge: what dry solids content is fed? What is volatile solids or ash content in the sludge before and after digesting? Is there mixing inside methane tank? What is hydraulic volume of methane tank (what volume of material can be filled to the empty tank)? Is the process continuous (constant feeding and constant discharging), or batch (one time feeding and one time discharging)? If the process is continuous: ñ ñ What volume of incoming material per day? What are flow rate and pollution variations of incoming material during day, week, month? If the process is batch: ñ How much time it takes from filling methane tank till the discharging of digested material? What is biogas yield (m3/day at normal conditions: 1 atm; 0 till 20 o C)? Does biogas contain enough methane to combust it? What percent of methane biogas consist of? Do they generate electricity from methane or use it only for heating? ñ If they generate electricity, what power (in Kilo-watts, or Mega-watts) generators they have, and what percent of their power can be used for electricity production daily (or how many KWh they produce per day)? Do they have problems with sulphur hydrogen in gas or sulphuric acid in generator or heater? Do they have problems with effluent or sludge disposal after anaerobic process? Do they have sludge dewatering equipment (centrifuge, filter press, belt press)? Do they have scaling or foaming problems in the process?

22 Summary. What we can do? If ph low (< 6,5), it means that methane tank is overloaded. We probably could help with bioproducts If effluent has too big COD or BOD (>1000 mg/l) after the process, probably we can help with bioproducts as well If effluent has too much suspended solids, we can help with flocculants and coagulants, but customer needs sedimentation or flotation unit for that If it takes too long (longer than foreseen in design) to digest sludge or treat effluent, probably we can help with bioproducts. If biogas yield or composition is unsatisfying, we could help with bioproducts If scaling or foaming appears anywhere in the process, we have efficient antiscalants or antifoamers If sludge dewatering unit they have we certainly can offer efficient flocculants Info: