Environmental Protection

City Saves With Industrial Process

Anaerobic process converts organics with less biosolids, odor, and energy cost

Escalating operational costs are forcing conventional wastewater treatment plants to try unconventional processes. A Cartersville Georgia Water Department plant, for example, is implementing a process that has been successful in industrial facilities but has never been tried in a full-sized municipal wastewater treatment plant. The treatment process is called the CATABOL ™ Process and relies primarily on the anaerobic conversion of organics. The plant -- the Biosolids Production Facility -- may need to be renamed upon project completion because the new process greatly reduces the amount of biosolids produced.

Process Principles
Both aerobic and anaerobic treatment processes start with hydrolysis and acidification to reduce complex organics to carbon dioxide, hydrogen, volatile fatty acids, and other simple organic compounds. From that point on, treatment can be accomplished either aerobically or anaerobically. When sufficient aeration exists, aerobic conversion of simple organics produces about 50 percent new biomass and 50 percent carbon dioxide. Without aeration, anaerobic conversion forms about 5 percent new biomass and about 95 percent gas, mainly methane (70 percent) and carbon dioxide (25 percent) as well as some nitrogen and other gases.

The differences in these products are significant. Aerobic biomass, which is prone to rot, needs digestion or other treatment for stabilization while strongly anaerobic (predominantly methanogenic) biomass does not rot. Aerobic biomass cannot be accumulated for future use without continuously providing food and aeration. Anaerobic biomass, on the other hand, is stable and dies off very slowly. Multi-year accumulations of anaerobic biomass are possible due to its slow metabolism. It can be revived rapidly by adding food and proper conditions.

The new process makes use of the interchange of aerobic biomass and strongly anaerobic biomass. The interchange is defined as the controlled transfer of anaerobic biomass from the sludge conditioner to the water train, and the transfer of aerobic biomass from the aerobic zone of the bioreactor in the water train to the sludge conditioner. This process was first developed and patented by Dr. Boris Khudenko in 1993. The interchange of biomass is possible without upsetting biological activity. Aeration of both types of biomass results in stripping carbon dioxide but is especially useful for anaerobic biomass because it improves settling characteristics. Iron and lime, which are largely recoverable, can be used to control alkalinity and pH of the biomass relatively inexpensively. By depriving sulfate-reducing bacteria of volatile fatty acids, the process minimizes hydrogen sulfide odors.

Treatment Process Units
The basic units of the new process include a bioreactor with sequential anaerobic and aerobic zones, a mainly aerobic polishing tank, a solids separator that can be a standard clarifier or membrane, a side-stream conditioner, and inert removal of solids. Methanogens are cultivated and stored in the sludge conditioner for controllable delivery to the water train.

Rapid hydrolysis and acidification occur in the anaerobic zone of the bioreactor. The methanogens and the aerobes quickly reduce the volatile fatty acids, thereby minimizing hydrogen sulfide production. As a result, Cartersville can configure its bioreactors and the sludge conditioner basins as open tanks with digestion gas release to the atmosphere.

The polishing bioreactor completes the process by reducing nutrients, virtually eliminating biochemical oxygen demand and drastically lowering chemical oxygen demand. The polishing stage also balances system biomass. Cartersville will clarify its treated wastewater using standard clarifiers followed by chlorination, dechlorination, and discharge.

Periodically, mixed liquor suspended solids from the bioreactor will be sent to the inert solids removal units, comprised of fine screens and specially designed hydrocyclones. Screenings will be pressed to about 40 percent solids using a conveyor with a press attachment. The cyclone slurry will be thickened to about 35 percent solids by gravity release of water. The pressed screenings and the thickened cyclone solids will be disposed at a sanitary landfill. The city expects that screenings and thickened cyclone solids will be the only solids removed from the plant.

Khudenko Engineering Inc. (KEI) has compared its process to using activated sludge with aerobic or anaerobic sludge digestion and biological processes with low or high methanogen-content sludge conditioner. Using the BioWin model, an internationally accepted tool for biological wastewater treatment process simulation, KEI found that the CATABOL Process and a strongly anaerobic sludge conditioner produce fewer solids and lower energy requirements. Clearly, the wastewater treatment plant will avoid costly treatment and disposal if it can avoid producing solids in the first place.

Plant Modification
The Cartersville plant, which serves a community of 19,000 people, is a 15-million-gallons-per-day (mgd) extended aeration plant. The modification will cost $6.7 million and will increase treatment capacity by at least 5 mgd. The increase in capacity will be accomplished by converting an existing primary clarifier to a final clarifier and an existing equalization basin to a polishing tank.

The existing plant has four 1.0-million-gallon aerobic digesters, which are being converted to sludge conditioners for production and storage of methanogens. An old circular tank will be converted to screen sludge from the bioreactor and to remove inert solids using the hydrocyclones. Solids then will be transferred to the sludge conditioner or sent back to the head of the plant. The existing plant has two anoxic tanks at the headworks, two 2.5-million-gallon aeration basins, and one 5-million-gallon aeration basin. The two smaller aeration tanks will be converted to predominantly anaerobic bioreactors. The existing 5-million-gallon tank will be converted to an aerobic polishing tank. The plant ultimately will have a split flow with a bioreactor, polishing tank, and clarification in each train.

System Benefits
Cartersville decided to modify its facility to take advantage of the benefits of the industrial process. Prior to making this decision, the city operated a 2,500-gallon pilot treatment plant for five months of 2003. The pilot digester was fed mixed liquor from the existing aerobic (extended air) system. Based on the promising pilot plant results, Cartersville is investing in the process because

  • Biosolids production is expected to be reduced by 75 percent to 90 percent when compared to conventional operation.
  • Electrical consumption should be reduced by about 70 percent (last year, the plant spent $576,000 on electricity).
  • Nutrients should be eliminated, which will lower chemical costs for nutrient removal.
  • The process is expected to save between $600,000 and $700,000 per year in operating costs, which is 25 percent of the facility's operating budget. The savings are expected to increase as future operations and maintenance costs increase.

One half of the modifications will be fully operational in June, and the entire project should be completed by December. While the plant is being modified, Cartersville has begun using the process and still has met all state-mandated permit requirements for its effluent (see Table).

Plant Performance Using CATABOL Process
Average Value (mg/L)
COD
Soluble COD
BOD
TSS
VSS
TKN
Ammonia
Phosphorous
Influent
582
250
242
277
205
56.1
28.6
21.4
Effluent
64
54
7
6
5
3.8
1
18.2
Removal
89%
78%
97%
98.9%
98%
93%
97%
15%
Permit Limit
**
**
30
30
**
**
10
**
Note: Monitoring period is from Sept. 9 to Oct. 30, 2006

COD: chemical oxygen demand.
BOD: biochemical oxygen demand.
TSS: total suspended solids.
VSS: volatile suspended solids.
TKN: total Kjedahl nitrogen.


The effluent's total suspended solids averaged 6 parts per million (ppm), and the biochemical oxygen demand averaged 7 ppm using secondary treatment. Completion of the plant modifications will allow more flexibility in operation. The plant is not removing phosphorous now, because there is no phosphorous limit in the current permit. When the project is completed, the plant will be able to achieve the expected future limit.

In 2005, the influent flow averaged 8.5 mgd, and the facility generated and disposed of 12,500 wet tons of solids (6 dry tons/day). The solids were dewatered using belt filter presses and transported to agricultural lands.

The conversion has already helped save the city of Cartersville $30,000 per month in dewatering and transportation costs. For the monitoring period Sept. 9 to Oct. 30, 2006, no solids have been wasted from the system. Reduced aeration requirements also have lowered electrical consumption by 50 percent. In the long term, solids generation is expected to be reduced by 85 percent to 90 percent, compared to conventional operation. When fully operational, plant performance and cost savings will be completely documented. However, the results achieved to date indicate that additional savings can be expected.

This article originally appeared in the January/February 2007 issue of Water & Wastewater Products.

This article originally appeared in the 01/01/2007 issue of Environmental Protection.

comments powered by Disqus

Free e-News Subscription

I agree to this site's Privacy Policy