From 1973 to 1976, the EPA and the state of California sponsored a pilot study in Carson, California. The final technical report was completed in 1980 and is available on the EPA’s website, entitled Waste Activated Sludge Processing (PDF), document EPA-600/2-80-147.
Several technologies were tested for the handling of waste activated sludge, but the process endorsed by the report was “that dissolved air flotation be adopted for thickening of waste activated sludge.” Both aerobic and anaerobic digestion were considered. However, aerobic digestion did not have a significant benefit and, due to the additional energy input required, was discarded in favor of anaerobic digestion, which also produces a usable source of energy.
Despite the findings of this report, some plants have chosen to skip the step of dissolved air flotation thickening and take their sludge straight to the digester without an additional thickening step.
The subsequent analysis compares the energy consumption of the thickening and digestion process with and without the addition of dissolved air flotation thickening.
Process flow diagram of typical wastewater treatment plant without waste activated sludge thickening:
Process flow diagram of typical wastewater treatment plant with waste activated sludge thickening:
Click images above to view full-size PDFs.
Generally, the temperature of the wastewater in any plant is dominated by the underground temperature in the region, which remains generally constant year-round for deeply buried pipes. Other factors may influence this water temperature, but once the water makes it underground, the temperature of the underground pipes will determine the temperature of the water.
Therefore the typical range of influent temperatures to wastewater treatment plants in the USA is:
Most of the country falls in the 50°F-60°F range.
For mesophilic digestion the target temperature is between 68°F and 108°F, with 98°F being the most typical target.
Heat is required in order to get from the wastewater temperature to the digester temperature. The amount of heating energy needed will be the critical component of the cost of using a digester without a DAF vs. using a DAF. Heating the thickened sludge will use substantially less energy than heating the unthickened WAS.
Water Specific Heat Capacity
Fortunately the specific heat of water doesn’t change much with temperature and ranges between 4.178 kJ/kg/°C and 4.205 kJ/kg/°C under the conditions usually seen in wastewaters. For the purposes of this analysis, we’ll use:
In order to do a cost comparison, we will need to put a price on both electrical power as well as natural gas, because the efficiency of heating using electrical power is very bad. A 2017 survey of commercial energy costs in the USA had a low of $0.075/kWh and a high of $0.29/kWh, with most utilities charging close to $0.10/kWh.
Typically natural gas is sold by $/MMBTU (dollars per million BTUs), the range for this unit of measure is $1/MMBTU to $6/MMBTU with an average of about $3/MMBTU. However, in order to have consistency between units, we’ll convert this to cost per kWh:
To do your own analysis using your local information, download this sample cost and ROI calculation spreadsheet.
The analysis will compare using the following equations. Note that we’ve assumed 3.785 kg/gal to be the density of water.
Test Case #1 (Average Case)
We’ll assume that the wastewater treatment plant produces 1000 gpm of waste activated sludge (WAS) in their secondary clarifier or clarifiers. Here we’ll assume the average temperature, natural gas, and electrical costs.
Without a DAF (or other sludge thickening)
Waste activated sludge goes from the secondary clarifier to the digester at a flow rate of 1000 gpm with a TSS concentration of 5000 mg/L.
The total energy cost for heating the water going to the digester is given by the following equation:
Note: The digester does produce heat from the biological processes, so the natural gas consumption will not really be that high. However, the amount of energy released will be the same for both the thickened and not thickened case, so the relative cost difference will be the same.
With a DAF
A 40’ diameter DAF with a 1200 gpm recycle is used to thicken the sludge from 0.5% (5000 mg/L) to 4% (40,000 mg/L). The flow going to the digester is Q=1000*0.5%/4%=125 gpm.
Estimated total installed cost of DAF system: $600,000.
Please contact WesTech to get a quotation on an appropriate DAF system for your needs.
The DAF system electrical loads are: Recycle Pump: 55 kW, Air Compressor: 5 kW, Drive Motor: 1 kW
Total energy cost:
- Total yearly savings: $439,756
- Estimated payback period: 1.36 years
- ROI based on a 20-year estimated life of DAF: $8.19 M
Test Case #2 (Pessimistic Case)
This test case represents a pessimistic scenario of the estimated savings. Calculations assume moderate electrical costs, cheap natural gas, a high water temperature to start, and the same sludge as Test Case #1.
Without a DAF
With the same DAF
- Total yearly savings: $81,021
- Estimated payback period: 7.4 years
- ROI based on a 20-year estimated life of DAF: $1.02 M
Even in the most pessimistic case, thickening with a DAF prior to feeding the digester will result in substantial energy savings and cost savings.