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Uplow Anaerobic Sludge Blanket Systems - UASBs An OnLine Primer Need to know more? Send us an email or feel free to search our online series with our give away search engine: |
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Let's calculate an estimate of the solids retention time for two UASB installations using the iterative formula - all data self-explanatory. Please be aware that these calculations are a simplification of the actual operating plant data. For example, actual solids inventories could be smaller or bigger and all other derived magnitudes adjusted accordingly. SRT is thus about ballpark.
| SRT Sludge Age Sizer | ||||||||||
| T | 20 | °C | ||||||||
| Yo | 0.2 | UASB | Average Flow | |||||||
| Kd | 0.024 | High Rate Anaerobic System | ||||||||
| flow in | 13233 | m3/d | 3.496 | mgd | ||||||
| CODin | 3805 | mg/L | 110813 | lbCOD/day | 50265 | kgCOD/day | ||||
| DAF rem | 0 | % COD pretreatment removal by DAF | ||||||||
| Percent | 100 | % COD goes to anaerobic | 50265 | kgCOD/day to anaerobic reactor | or | 3805 | mg/L | |||
| Percent | 87 | % sCOD removed | ||||||||
| Vreactor | 2400000 | gallon | 9084 | m3 | (from order of magnitude sizing) | |||||
| 2.4 | mg | 5.53 | kgCOD/day per m3 | |||||||
| Target MLVSS | ||||||||||
| 35000 | mg/L | 699720 | lb bugs | 317391 | kg bugs | |||||
| SRT | 91.78 | days | (iterates automatically) | f/m | 0.158 | kgCOD/d per kg bugs | ||||
| Yn = | 0.079 | |||||||||
| biomass yield = | 3458 | kg/day bugs | ||||||||
| HRT = V / Q = | 0.7 | days | ||||||||
| 16 | hours | |||||||||
As said before, please be aware that these calculations are a simplification of the actual operating plant data. For example, actual inventories (both bio and non bio) would be smaller or larger and all other derived magnitudes adjusted accordingly. SRT is thus about ballpark.
| SRT Sludge Age Sizer | ||||||||||
| T | 20 | °C | ||||||||
| Yo | 0.15 | UASB | ||||||||
| Kd | 0.024 | High Rate Anaerobic System | ||||||||
| flow in | 860 | m3/d | 0.227 | mgd | ||||||
| CODin | 3400 | mg/L | 6435 | lbCOD/day | 2919 | kgCOD/day | ||||
| DAF rem | 0 | % COD pretreatment removal by DAF | ||||||||
| Percent | 100 | % COD goes to anaerobic | 2919 | kgCOD/day to anaerobic reactor | or | 3400 | mg/L | |||
| Percent | 90 | % sCOD removed | ||||||||
| Vreactor | 210000 | gallon | 795 | m3 | (from order of magnitude sizing) | |||||
| 0.21 | mg | 3.67 | kgCOD/day per m3 | |||||||
| Target MLVSS | ||||||||||
| 6000 | mg/L | 10495.8 | lb bugs | 4761 | kg bugs | |||||
| SRT | 16.52 | days | (iterates automatically) | f/m | 0.613 | kgCOD/d per kg bugs | ||||
| Yn = | 0.110 | |||||||||
| biomass yield = | 288 | kg/day bugs | ||||||||
| HRT = V / Q = | 0.9 | days | ||||||||
| 22 | hours | |||||||||
In general one would want far more information as regards the application. Say for instance we want to design a high rate system for a starch processing plant. One would ask say
1. What’s the nature of the starch base? tapioca? maize? potato?..other?
2. Are we talking about native starch? Or modified starch production? The latter may lead to toxicity for an anaerobic system.
3. What’s the TSS contents of the waste water?
4. What is the minimum and maximum temperature of the waste water stream? For instance 20°C would be too low for a high rate anaerobic treatment without pre-heating of the waste water.
In general one requires as much information as possible about the wastewater characteristics including COD, TSS, VSS, alkalinity, pH, VFAs, and temperature in the reactor. Also, a schematic diagram of the reactor would be helpful. Lemon processing wastewaters often contain limonene, which is an organic solvent and can cause damage to anaerobic granules. Any information on limonene concentration in the wastewater will be helpful.
A startup of a granular sludge reactor at a paper mill in Canada took about one year to get a full inventory of biomass even considering that they purchased some granular sludge for startup. So one can expect it will take say, in absence of much further information, at least six months to get a full inventory of granular sludge.
Question: Is the development of this granular sludge possible onsite/locally?
Answer: Yes it is possible, but the rate and amount of granule formation depends on the type of wastewater being treated, the solids retention time, and the efficiency of the gas-liquid-solids-separator in the UASB reactor. Carbohydrate and alcohol wastes are best for granule formation, while proteins and organic acids do not form good granules. Solvents and oils, such as limonene, can interfere with granule formation.
Question: How long would it take to develop granular sludge from anaerobic sludge coming from other anaerobic digestion technologies? Other ways to develop the granular sludge?
Answer:
If the net yield is 0.14, as for carbohydrates when operating at an SRT of 30 days, and if 50% of the biomass forms granules, and the COD loading rate is 10 g/L/day, granule formation would be approximately (0.14*0.5*10 = 0.7 g/L-day or 700 mg/L-day). And if the biomass concentration in the reactor is 40,000 mg TSS/L, granules would grow at a rate of 1.75%/day.
For organic acids wastewater, the net yield decreases to 0.03 g VSS/g CODr, so that granule formation would be only 1.75*0.03/0.14 = 0.38%/day. There is no other way to produce granules except in a UASB or EGSB reactor. Upflow velocities also affect the rate of granule retention, so that high COD wastewaters ( COD > 10,000 mg/L) produce granules at a higher rate than low COD wastewaters ( COD ~ 3000 mg/L).
Question: If sludge from other anaerobic digestion technologies is available, is it possible to start up a UASB/EGSB reactor? Should they work in a different way than the design one? Could they be developing the sludge and at the same time treating the effluent with the same performance of design?
Answer: It is possible to start up a UASB reactor using non-granular sludge, but it will take several weeks or months for granules to form. Upflow velocities should be as low as possible when using non-granular sludge to start up a UASB reactor.
