Anaerobic Ponds

 

Anaerobic ponds are one of the simplest, most beautiful wastewater treatment systems known to civilisation, because the wastewater essentially treats itself. All we need to do is give it a place to work. If only the rest of life was so easy.

 

 What size should an anaerobic pond for a high strength organic waste be?

 

First we need to know:

1) How much effluent do we need to treat,

2) What the BOD of the effluent is,

3) How warm or cold is it where the anaerobic pond will be, and

4) How well do we want to treat the waste.

 

The design of an anaerobic pond has to be based on several factors besides the ones above; including the type of wastewater, the nutrients in the wastewater, the consistency of the flow and the consistency of the strength of the flow. Anaerobic ponds sometimes also generate odours so it is important to be aware of the local community, as well.

 

For the purposes of this example we will assume that the high strength organic waste is susceptible to anaerobic decomposition (as most are), there are plenty of nutrients, that the waste strength and flow is consistent and that there are no local residents to be bothered by odours.

 

The answers to the questions above for this example are:

1) 1 Megalitre (ML) / day, 7 days per week

2) The average BOD5 is 2000 mg/L

3) The anaerobic pond will be in Wagga Wagga in southern New South Wales where the winter air temperature averages about 8o C. (Mean maximum temperature = 13.5oC and mean minimum = 3.3oC in June, July and August)

4) We would like to treat to 85 or 90% so that we can get the BOD down to 200 or 300 mg/L. After that it can be ready to be polished up with a little aerobic / facultative treatment.

 

Due to the many different types of wastes and the impact of temperature on the bacterial activity (that is the workhorse of the treatment), the loading rates for anaerobic ponds vary gigantically. A review of my library gives an illustration as shown in the Table at the bottom of the page.

 

Example Calculations

 

For this example we will use the two main methods of anaerobic pond design, detention time and loading rate, together. We will try a loading rate of 0.07 kg/m3-day and a detention time of 35 days to reflect the cool winter climate at Wagga Wagga.

 

Volume  = Flowrate x Detention time = 1 ML/day x 35 days = 35 ML = 35,000 m3

 

Volume  = BOD Mass Loading / BOD Mass Loading Rate

BOD Mass Loading = 1 ML/day x 2000 mg/L x 106 L/ML x 10-6 kg / mg = 2000 kg BOD / day

 

Volume = 2000 kg BOD / day / (0.07 kg BOD / m3 – day) = 29,000 m3

 

Using these two results we can assume that a volume of 30,000 m3 will be appropriate. In order to be sure that we don’t get short circuiting and to add flexibility to the operation it will be best to have two ponds with a 3:1 length to width ratio. The recommended depth is between 2.5 and 6 metres. The cheapest and most effective is usually about 5 metres.

 

Area of each pond = 30,000 m3 / 5m deep x 2 ponds = 3000 m2 for each pond

 

Area = width x length

Length = 3 x width

Area = width x (3 x width) = 3000 m2

width = (3000 / 3)0.5 = 10000.5 = 32 metres

length = 3 x width = 96 metres

 

So the anaerobic pond treatment of the 1 ML of 2000 mg/L wastewater can be accomplished using two ponds in series, each 32 metres wide, 96 metres long and 5 metres deep.

 

 

 


Range of Loading Rates for Anaerobic Ponds from Various Sources

 

Reference

Detention time (days)

Listed Loading Rate

 

Converted Loading Rate

(kg/m3-day)

Depth

(metres)

Comments

Barnes, Bliss, et al (1981)

8 to 40

25 to 40 grams/m2 day

(3.75m depth)

0.007 to 0.011

2.5 to 5.0

Primarily for medium strength domestic sewage

Metcalf and Eddy (1979)

5 to 50

200 to 500 kg/ha-day

(3.75m depth)

0.005 to 0.015

2.5 to 5.0

Primarily for medium strength domestic sewage

Eckenfelder

(1980)

5 to 50

250 to 4000 lbs BOD per acre-day (11.5 ft)

0.008 to 0.130

2.4 to 4.6

Broad range for all applications

Corbitt (1989)

1 to 50

0.05 to 0.25

kg/m3-day

0.05 to 0.25

2.4 to 6.1

Loading “widely varying due to wastewater characteristics”

 

It is important not to confuse anaerobic pond loading rates with the loading rates for anaerobic digesters. Anaerobic digesters breakdown sludges and their loading rates are usually in the 3 to 10 kg of Volatile Solids / m3 – day.

 

Barnes, D, PJ Bliss, BW Gould and HR Valentine (1981) Water and Wastewater Engineering Systems, Longman Scientific and Technical, Essex

Corbitt, Richard A. (1989) Standard Handbook of Environmental Engineering, McGraw-Hill, New York

Eckenfelder, Jr., W. Wesley, (1980) Principles of Water Quality Management, CBI Publishing Company, Boston

Metcalf and Eddy (1979) Wastewater Engineering: Treatment, Disposal, Reuse, McGraw-Hill, New York, page 553