Responsible Sewage Treatment Victoria

Energy Use by Sewage Treatment Plants

" a future Victoria treatment plant would use about as much energy as 1,000 to 1,500 average BC homes. "

"The sludge management system that is selected could have a large impact on energy consumption."

"I think the biggest environmental downside of the proposed plant by far will be off site sludge management, whether it is lime stabilization or composting followed by use on farmland, spray irrigation on farm or forest land, or disposal at the Hartland Landfill."

The following comments were prepared for Responsible Sewage Treatment Victoria

May 24, 2007

Comments prepared by T. R. Lidkea, M.A.Sc., Environmental Engineer

Overview

Energy is only one of the costs associated with building and operating a treatment plant. Costs of land, labour, materials, and chemicals should obviously be considered as well, in the context of the overall economics of a treatment plant. There are tradeoffs between different types of costs. For example, more expensive, higher efficiency pumps or aerators, or large facultative treatment lagoons requiring a very large area, would reduce energy costs. Consideration should be given to all the local and possible future cost components of a treatment plant.

I have no particular problem with energy consumption per se, but am strongly opposed to the large scale expenditure of economic resources, in the case of the proposed treatment plant, for negligible benefit. This is called waste. However, the following comments focus on energy use.

Whatever future treatment plant option(s) and location(s) are selected to serve the core Victoria area, the new system(s) would need some combination of influent and/or effluent pumping and headworks/screening pre-treatment similar to what is done now. It is likely that a future plant (or multiple plants – for simplicity let’s just use the singular from now on) would require additional energy use in the upstream collection and trunk sewer works, but without knowing the location and elevation of the mythic plant, we don’t know whether this additional collection-related energy use would be large or small. Therefore, I suggest that for the purpose of public debate, it would be appropriate to simply focus on the additional energy requirements of the future treatment plant.

The major additional treatment-related energy uses would be:

· Primary tank sludge management, i.e. pumps and scrapers;

· Aeration and/or mixing in secondary treatment tanks, or power to turn Rotating Biological Contactors (or whatever means is adopted to aerate the wastewater), and related sludge recirculation pumping and transfer pumping;

· Sludge management, for which there are many options: 1) Dewatered un-digested sludge can be composted or incinerated or stabilized by some other means; 2) Anaerobic digestion, usually used for larger plants, followed by dewatering (or not) and land-spreading or disposal; 3) Aerobic digestion, either auto-thermophillic or not, again followed by dewatering (or not), and use or disposal. The choice of sludge management option could have a significant impact on energy use.

Incidentally, I think we have land-based treatment now, to wit preliminary treatment, consisting of fairly fine screening (6 mm) at both Clover Point and Macaulay Point.

Energy Consumption Estimates

Estimates of the additional energy use for a future Victoria treatment plant are based on a review of the report Energy Consumption Implications for Wastewater Treatment in Canada, prepared by Hydromantis Inc. and XCG Consultants Ltd. for Environment Canada, and submitted on March 29, 2006.

Energy estimates from the report are based in part on assumed wastewater BOD₅ and TSS levels of 200 and 220 mg/L respectively. Treatment plant flows are categorized for plants ranging from “Very Small” to “Very Large”. Victoria’s plant would fall into the “Very Large” category.

The report briefly discusses the potential variation in energy use across Canada, concluding that there is little basis for assuming different energy consumption based on location. It may be that treatment plants in cold, interior areas use more energy than plants in coastal areas, but this is likely offset by the increased precipitation in the coastal areas, leading to higher rates of infiltration and inflow into sewers, and thus higher pumping costs. I believe it would be valid to compare energy use for “Very Large” plants from elsewhere in Canada to the possible future energy use of a “Very Large” plant in Victoria.

Although the exact type of treatment and the unit processes have not yet been selected for a plant to serve the Victoria area, it is likely that the Victoria plant would include primary treatment by settling, some variant of activated sludge secondary treatment, and some type of thickening or dewatering of primary and secondary sludge. I think the best approach would be to define a range of energy uses by existing Canadian treatment plants that are similar to one that Victoria could be forced to build.

The Hydromantis report presents the energy use of twenty-four treatment plants, including fourteen non-nitrifying conventional activated sludge treatment plants, similar to the foregoing “assumed” Victoria plant. I have assumed that a treatment plant serving Victoria would not need to provide for nitrogen removal, but then again, nitrogen removal is no more ridiculous in the context of Victoria’s long deep outfalls than is BOD removal. If politicians decide they would like nitrogen removal, or phosphorous removal, or even distillation or reverse osmosis, then the energy use would be significantly higher than the following estimates.

From Table 50 of the Hydromantis report:

The average energy use of these three plants (similar in size and type to what Victoria might build) varies from 69 to 189 kWhr/year per m³/day. The average is 126 kWhr/year per m³/day. At an estimated average flow of 120,000 m³/day for the Victoria core area, the expected energy use would be approximately 15,120,000 kWhr/year.

To put this is a more readily understood context, a typical household in British Columbia consumes 10,000 to 12,000 kWhr/year. Therefore, the proposed Victoria treatment plant would use the equivalent amount of energy as 1,260 to 1,510 households.

If anaerobic sludge digestion is selected as part of the sludge management program, the plant’s energy requirement could be reduced by 10% to 40%. (According to the Hydromantis report, the three plants listed in the table above could reduce their energy requirements by 20%, 10% and 40% respectively by converting methane gas to electricity for use on site). Assuming an average reduction of about 23%, the Victoria treatment plant’s energy requirement could be reduced to about 11,642,000 kWhr/year, or the energy consumption equivalent of approximately 970 to 1,160 average households.

For the sake of simplicity, I think it would be reasonable to say that a future Victoria treatment plant would use about as much energy as 1,000 to 1,500 average BC homes.

The anaerobic production of methane gas to produce electricity is not reliable and requires a backup power system. The potential to reduce energy cost is only one of the factors that might favour the choice of anaerobic digestion. Other options, such as aerobic digestion, or heat/lime stabilization or composting of undigested sludge, may be favoured for overall economic reasons even though they do not produce methane.

The sludge management system that is selected could have a large impact on energy consumption. The Hydromantis report does not assess the energy requirements of off site sludge transportation and use or disposal. In the CRD, these costs could be significant. The Peninsula Wastewater Treatment Plant adopted a system using heat plus low pH lime stabilization of sludge with the expectation that it could be used on Saanich Peninsula farmland. This use option did not materialize, and to date the lime-stabilized sludge has mainly been disposed of at the Harland Landfill. It seems that the major difficulty was in finding enough farmland and enough willing farmers to accept the stabilized sludge product. Some of the peninsula sludge is being used to prepare a topsoil product for use on new developments, but this option is reportedly coming to an end.

Even though the energy consumption of the proposed Victoria treatment plant would be wasted in terms of producing anything useful in the way of environmental protection, I do not see this consumption as a significant environmental problem in and of itself. I think the biggest environmental downside of the proposed plant by far will be off site sludge management, whether it is lime stabilization or composting followed by use on farmland, spray irrigation on farm or forest land, or disposal at the Hartland Landfill.