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Anaerobic Systems

An OnLine Primer

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ANAEROBIC TREATMENT CASE STUDIES

Let's calculate an estimate of the solids retention time for a number of anaerobic 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 larger and all other derived magnitudes adjusted accordingly. SRT is thus about ballpark.

 

SRT Sludge Age Sizer
T 20 °C  
Yo 0.2 EGSB Average Flow
Kd 0.024 High Rate Anaerobic System
flow in 18526 m3/d 4.895 mgd
CODin 5221 mg/L 212870 lbCOD/day 96557 kgCOD/day
DAF rem 0 % COD pretreatment removal by DAF
Percent 100 % COD goes to anaerobic 96557 kgCOD/day to anaerobic reactor or 5221 mg/L
Percent 67 % sCOD removed
Vreactor 2055000 gallon 7778 m3 (from order of magnitude sizing)
2.055 mg 12.41 kgCOD/day per m3
Target MLVSS
24000 mg/L 410835.6 lb bugs 186354 kg bugs
SRT     21.00 days (iterates automatically) f/m 0.518 kgCOD/d per kg bugs
Yn = 0.137
biomass yield = 8876 kg/day bugs
HRT = V / Q = 0.4 days
10 hours

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

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.

TROUBLE CONSTITUENTS

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.

STARTUP TIMES

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.

COMMON REACTOR QUESTIONS & TROUBLESHOOTING


Consider a facility that produces fresh citrus fruit and processed fruit. Say today one of the companies needs almost 70 tons of granular sludge.

LOCAL SUPPLY

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.


TIME REQUIRED

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).


MAKE-SHIFT seed source ALTERNATES

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.


ASB

Willie

Need Some Help? answers@engineeringfundamentals.com - James C. Young Environmental - Balestie, Irwin & Balestie