American Water College 2010

Transcription

1 Vocabulary Anaerobic Sludge Digestion Aerobic Digestion The breakdown of wastes by microorganisms in the presence of dissolved oxygen. This digestion process may be used to treat only waste activated sludge, or trickling filter sludge and primary (raw) sludge, or waste sludge from activated sludge treatment plants designed without primary settling. The sludge to be treated is placed in a large aerated tank where aerobic microorganisms decompose the organic matter in the sludge. This is an extension of the activated sludge process. Anaerobic Digestion Wastewater solids and water (about 5% solids, 95% water) are placed in a large tank where bacteria decompose the solids in the absence of dissolved oxygen. At least two general groups of bacteria act in balance: (1) Saprophytic bacteria breakdown complex solids to volatile acids, the most common of which are acetic and propionic acids; and (2) Methane Fermenters breakdown the acids to methane, carbon dioxide, and water. Alkalinity The capacity of water or wastewater to neutralize acids. This capacity is caused by the waters content of carbonate, bicarbonate, hydroxide, and occasionally borate, silicate, and phosphate. Alkalinity is expressed in milligrams/liter of equivalent calcium carbonate. Alkalinity is not the same as ph because water does not have to be strongly basic (high ph) to have a high alkalinity. Alkalinity is a measure of how much acid must be added to a liquid to lower the ph to 4.5. Buffer Capacity A measure of the capacity of a solution or liquid to neutralize acids or bases. This is a measure of the capacity of water or wastewater for offering a resistance to change in ph. Mesophilic Bacteria Medium temperature bacteria. A group of bacteria that grow and thrive in a moderate temperature range between 68 o F (20 o C) and 113 o F (45 o C). The optimum temperature range for these bacteria in anaerobic digestion is 85 o F (30 o C) to 100 o F (38 o C). Psychrophilic Bacteria Cold temperature bacteria. A group of bacteria that grow and thrive in temperatures below 68 o F (20 o C). Thermophilic Bacteria Hot temperature bacteria. A group of bacteria that grow and thrive in temperatures above 113 o F (45 o C). The optimum temperature range for these bacteria in anaerobic decomposition is 120 o F (49 o C) to 135 o F (57 o C). Aerobic thermophilic bacteria thrive between 120 o F (49 o C) and 158 o F (70 o C)

2 Seed Sludge In wastewater treatment, seed, seed culture or seed sludge refers to a mass of sludge which contains populations of microorganisms. When a seed sludge is mixed with wastewater or sludge being treated, the process of biological decomposition takes place more rapidly. Supernatant Liquid removed from settled sludge. Supernatant commonly refers to the liquid between the sludge on the bottom and the scum on the surface of an anaerobic digester. This liquid is usually returned to the influent wet well or the primary clarifier. Purpose of Anaerobic Digestion To convert wastewater solids to a relatively odor free sludge that is capable of being dewatered and disposed of without causing a nuisance. Reduction Efficiencies Volatile solids reduction in a properly operated digester is 50%-60% Process Description Organic solids contained in the sludge are broken down by mesophilic bacteria which reduces the total volume of sludge requiring disposal. Bacteria we call acid formers convert the organic solids to volatile acids. Then, a second group of bacteria we call methane fermenters convert the volatile acids to methane gas. There are other byproducts of this process such as carbon dioxide, and hydrogen sulfide. Under optimum conditions (ph / temperature o F / proper vol. acid/alk ratio) the digester will reduce the volatile organic solids by 50-60% in about days. Process Control Proper Mixing is essential: 1. utilizes entire volume of digester 2. distributes raw sludge throughout digester 3. allows contact between bacteria and undigested sludge (food) 4. controls ph by distributing buffering alkalinity throughout digester 5. maintains even heat distribution 6. minimizes grit settling on tank bottom and the formation of a scum layer on top

3 Feeding the Digester 1. Pump as thick as sludge as possible 4-6% if possible (minimize water content) 2. Thicken sludge if necessary (DAF, Gravity, etc.) 3. frequent small meals are better than infrequent large meals 4. loading rate should be lbs VS/ft 3 of heated digesting sludge Maintain Volatile Acid to Alkalinity Ratio 1. Monitor VA/ALK regularly (should remain < 0.1) 2. Take action if ratio increases 3. VA/Alk is first sign of trouble in a digester Struvite Control 1. Magnesium Ammonium Phosphate (hard scale that can clog digested sludge piping) 2. Controlled by precipitating Phosphate with Ferric Chloride Possible Modes of Operation (based on temperature range) 1. Psychrophilic a. Cold temperature (50 60 o F) b days required c. Very uncommon design 2. Mesophilic a. Medium temperature ( o F) b days at 95 o F c. Most common design 3. Thermophilic a. High temperature ( o F) b days required c. Few plants operate in this range Sludge Digestion Facts 1. produces 8 12 ft 3 of gas for every 1 pound of volatile matter added 2. produces ft 3 of gas for every 1 pound of volatile matter destroyed 3. Methane (CH 4 )= 65-70% 4. Carbon Dioxide (CO 2 ) = % % of gas contains nitrogen, hydrogen, hydrogen sulfide 6. If air enters the digester, an explosive mixture of gases can result

4 Primary Sedimentation Basin raw sludge Generic Anaerobic Digester Why digest sludge? 1. stabilize organic matter 2. reduce mass and volume of waste solids sludge gas heater/mixer * The digester can only reduce Volatile Solids* Digested sludge Organic loading is determined by calculating lbs of Volatile Solids pumped to the digester each day for each cubic foot of digester space. Example: If a plant pumps 4,000 gallons of sludge to the digester, how many pounds of volatile solids were pumped to the digester given the lab data? TS = 6% VS = 72% Digester Gallons = 100,000 Solids lbs = (gallons pumped) x (8.34) x (Total Solids) x (Volatile Solids) = (4,000) x (8.34) x (0.06) x (0.72) = 1,441 lbs Volatile Solids/day 100,000 gallons / 7.48 gal/ft 3 = 13,369 ft 3 1 ft 3 = 7.48 gallons Loading = 1,441/13,369 = 0.1 lbs VS/ft 3 /day

5 Process Control Guidelines Parameter Range Primary Sludge % Total Solids (TS) 3% - 6% Primary Sludge % Volatile Solids (VS) 70% 78% Digester Volatile Acids (VA) mg/l as acetic acid Digester Alkalinity (ALK) 2,500 3,500 mg/las calcium carbonate VA / ALK ratio < 0.1 Digester ph Carbon Dioxide (CO 2 ) 30% - 35% Methane (CH 4 ) 65% - 70 % Gas Production ft 3 for each pound VS destroyed % Volatile Solid Reduction 50% 60% Organic Loading Rate lbs VS/day/ft 3 Temperature Range o F Volatile Acid Determination This test is used as a process control measurement. It, when combined with Alkalinity, will give an indication of a digester s health, as well as function as an indicator of a problem. Volatile acids can be removed from sludge that is being digested by a distillation process. These acids can then be titrated with a base to determine the concentration. A normal range for a properly operating digester is mg/l as acetic acid. A sudden increase in the volatile acid concentration is an indication that the digester is going sour. Procedure: 1. collect a representative sludge sample 2. separate the supernatant from solids in centrifuge 3. collect 100 ml of supernatant and place in distillation flask 4. add 100 ml of distilled water and 5 ml of sulfuric acid 5. begin distilling but discard the first 15 ml of distillate 6. collect 100 ml of distillate and titrate with sodium hydroxide to phenolphthalein endpoint (ph 8.3) 7. Calculation - mg/l acetic acid = ml NaOH x N x 60,000 ml sample x f f = distillation recovery factor

6 Alkalinity Determination Alkalinity is the ability for the digester to resist a change in ph. Since the digestion process produces acid, there needs to be adequate buffering capability to prevent a sour digester. Typical alkalinity for a healthy digester is 2,500 3,500 mg/l as calcium carbonate. Procedure: 1. collect a representative sludge sample 2. separate the supernatant from solids in centrifuge 3. pipette 10 ml of supernatant into beaker 4. add 90 ml of distilled water 5. titrate with acid to ph of Calculation - mg/l alk as CaCO 3 = ml acid x N x 50,000 ml sample Volatile Acids/Alkalinity Ratio 1. Divide result of Volatile Acid by result of Alkalinity 2. Result should be 0.1 or less (higher indicates a problem) 3. At 0.5 the gas composition will change ( more CO 2 and less CH 4 ) 4. At 0.8 the ph will begin to drop Curing a Sour Digester 1. Reduce feeding rate or stop if possible for a few days to allow digester to recover 2. Increase mixing of sludge to ensure methane formers are getting fed (vertical mixing-bottom to top) 3. Pump seed sludge from secondary digester to sour digester 4. Chemical Addition as last resort a. Lime will increase solids load b. Soda Ash more expensive but adds less solids load c. Add lime through scum box 5. Calculating Lime dose Add enough lime to neutralize the volatile acids 1 mg/l = 1 mg/l Determine pounds of volatile acids in digester and add an equivalent weight of lime

7 Example: If the digester is upset and the volatile acid concentration is 350 mg/l, calculate how many pound of lime are required to neutralize the digester. Digester contains 180,000 gallons of sludge. Lbs = (MG sludge ) x (mg/l VA) x (8.34) = (0.18 MG) x (350 mg/l) x (8.34) = 525 lbs Foaming Digester 1. Result of active gas production during initial digestion stage 2. Caused by overfeeding 3. Prevented by adequate mixing during start-up Component Digester Equipment Purpose 1. Sludge Feed Transports sludge to digester for treatment 2. Draft Tube Directs raw and treated sludge to heating zone 3. Gas Injectors Inject gas into sludge for mixing and to aid heat transfer 4. Hot Water Jacket Contains hot water for sludge heating 5. Deflection shield Deflects heated sludge throughout digester 6. Gas Mixer Pumps gas from gas draw-off to gas injectors 7. Gas Draw-off Draws gas from top of digester to gas system 8. Hot Water Piping Transports hot water from heat exchanger to hot water jacket 9. Sludge Draw-off Removes digested sludge from bottom of digester 10. Supernatant Draw-off Removes supernatant from different levels of the digester 11. Supernatant Box Collects supernatant for transport to plant recycle flow 12. Water Seal Prevents explosive condition (keeps gas in and air out) 13. Vacuum Relief Protects digester cover from collapse in event of vacuum 14. Pressure Relief Relieves excessive pressure so water seal is not blown out (6-8 inches of water) 15. Flame Arrester Prevents spark or flame from entering the digester 16. Sediment Trap Traps sediment in digester gas 17. Condensate Drain Drains condensate removed from digester gas 18. Gas Piping Transfers digester gas from digester to gas system

8 - 60 -