The cost of treating sewage 2

[Jbuer]

I'm curious about what factors affect the cost of treating wastewater. I live in a Canadian city of about 650,000 people. Based on numbers that are easily available I figure something in the order of 50% of the wastewater the city treats is rainwater likely mostly from combined sewers and weeping tiles. I'm wondering in general terms how much could the city save in treatment costs and capital costs in future treatment plants if say that 50% could be reduced to say 25%? Is there a downside to reducing the clean water portion too much?

 

[bimr]

The cost of treating wastewater depends on the degree of treatment. If you have stringent discharge limits, the cost of treatment increase.

A large part of the cost of treatment is the cost of pumping. Anything that can be done to remove clean water like rainwater helps to reduce the pumping cost. Most municipal agencies require separate sewers so that rainwater is not moving through the treatment plant. One of the big problems with rainwater is that it increases the peak loading on treatment plants. Sizing a treatment plant for peak loading is a costly capital expensive. At the same time, the ability to achieve effective treatment at the peak conditions is very difficult. A large municipality may have a 4:1 peak to average flow. A large municipality with a combined flow may have a 8-10:1 peak to average flow.

There is little to no downside with removing the clean water portion.

 

[cvg]

constructing a separate stormwater drainage system is expensive. and stormwater that runs off roadways may need treatment also. those are your downsides

 

[gerhardl]

In my home country Norway (Scandinavia) there is set down building regulations where all new building construction/buildings have to plan and provide for surface draining of stormwater for the finished site. The amount required drained takes future climate changes (more surface water) into consideration, and might often require lower surface spaces[/(parks, playing grounds etc) as possible short-time surface reservoirs to avoid draiage systems overload. Surface water is for new constructions kept apart from wastewwater cleaning systems, tried to be absorbed in ground or drained naturally. Canada?

 

[Jbuer]

In Canada for about 50 years wastewater systems have separated sewage and rainwater. More recently many cities are trying to keep more of the rain from running off and contaminating rivers and lakes with still not very clean water.
I was hoping there might be rough rule of thumb that say a 10% increase in water volume might increase treatment cost say 2% to 5%. There is the extra cost in the capital cost of the plant, pumps and electricity but does the added "clean" water when it rains add substantially to cost of treatment?

 

[bimr]

There is the extra cost in the capital cost of the plant, pumps and electricity but does the added "clean" water when it rains add substantially to cost of treatment?

The cost of treatment consists of the amortized capital cost and the operating cost. If you have combined sewers, the treatment plant may be as 2 - 1/2 times larger. Power cost is the largest factor in operating cost and with larger aeration basins, the operating cost will be much higher.

In addition, the treatment efficiency will suffer. For example, if the treatment plant is removing phosphorus, there may not be enough waste strength to enable the phosphorus treatment to be successful. For example, the large wastewater treatment plant in Chicago (combined sewers) imports high strength brewery waste because the waste strength is too low for phosphorus treatment.

It is not possible to come up with a rule of thumb because there is a large variability with flows aa well as collection system infrastructure. Some sewer collection systems may have high storm flows from leaky sewers while other systems may have combined sewers.

Link

 

[SwinnyGG]

in Chicago (combined sewers) imports high strength brewery waste because the waste strength is too low for phosphorus treatment.

Off topic but this sentence made me curious.

Am I interpreting what you're implying correctly - that for removing phosphorous some minimum ppm threshold must be met in the feedstock to 'kick start' some other chemical process and allow separation to work efficiently?

This is way out of my wheelhouse, it's just interesting.

 

[gerhardl]

Jbuer:
In my experience your question is probably based on the wish to solve a certain technical/economical problem. In case, it is often sensible to present the original question, not only the way your mind tricks you to believe it should be solved. So, what is your real problem?

Within certain limits (equal or similar municipalities, climates, geographical layout, built areas, drainage and wastewater systems, technical cleaning methodes, wastewater composite, plant capacity according to flow variations etc. etc.) you could probably get a rough estimate of your factor within certain capacity limits.

As a far broader view: At the other end of the scale you will in primitive countries have to minimize cost by trying to avoid building in areas dexposed to danger, as the as the only affordable and cheapest solution.

For the most advanced societies worldwide the question is to allow a new area to lead surface drainage into an excisting plant, which would probably increase the cost, and should generally be avoided.

SwinnyGG: In general 'yes'. Example: some societies use grinded kitchen waste in home drainage, and have chemical system adapted to this, and thus depending on a certain wastewater solution. The other side is wastewater without this soup, where processes will need a not to 'thin' soup to work properly.

 

[bimr]

Am I interpreting what you're implying correctly - that for removing phosphorous some minimum ppm threshold must be met in the feedstock to 'kick start' some other chemical process and allow separation to work efficiently?

Since phosphorus is very harmful when released into the environment, many of the wastewater treatment plants are now required to remove phosphorus. Biological phosphorus removal is the most practical. In wastewater treatment with biological phosphorus removal, phosphate-accumulating organisms are used to remove the phosphorus. Infiltration/ Inflow or combined sewers can dilute the organic matter in the raw wastewater resulting in an insufficient supply of Volatile Fatty Acids to the phosphate-accumulating organisms. Higher flow rates associated with Infiltration/ Inflow or combined sewers can reduce the hydraulic detention time in the anaerobic selector. High flow rates can result in the loss of solids from the clarifiers. This will increase effluent phosphorus because of the phosphorus stored in the biomass. Loss of solids may also result in the washout of phosphate-accumulating organisms from the plant because of their relatively slower growth rate

 

[Jbuer]

Thanks for the comments. I'm not trying to solve a problem but hoping to improve my understanding of how clean water impacts the cost of water treatment. Our city is very slowly separating combined sewers and wondered what the city might save (ballpark) in treatment cost if more effort was made to reduce the clean water component.
Like I said mostly just curious. Thanks