Good morning.
Thank you for this opportunity to address the committee on behalf of the Canadian Water and Wastewater Association. I hope my remarks are helpful.
Good morning. Thank you for this opportunity to address the committee on behalf of the Canadian Water and Wastewater Association. I hope my address will be of use to you.
I'll be talking, primarily from the perspective of a municipal engineer, about the implications for phosphorus of the effective treatment of waste water.
As has been mentioned, phosphorus is an essential nutrient that supports the growth of algae and other biological organisms. Algal blooms are undesirable because of the potential for the production of toxins that are dangerous to humans, livestock, and wildlife. Fortunately, modern drinking water purification systems can effectively remove these toxins, and in the case of the Ottawa River, the presence of algal toxins in the incoming water from the Ottawa River has never been detected.
A second problem with algal blooms is that when the algae die off, the decomposition process depletes the water of oxygen. This can result in fish kills. This process is known as eutrophication.
For these reasons, it is important to control the amount of phosphorus that enters surface waters from municipal waste water treatment plants and natural surface runoff.
The amount of phosphorus that can be discharged into a given water body, without triggering algal blooms, is dictated by its assimilation capacity. Assimilation capacity is affected by a number of factors, such as the physical size of a lake and the flow rate of a river.
For example, the Ottawa River has significant assimilation capacity. It is large and doesn't have high background levels of phosphorus. For this reason, the discharge criterion for the city's waste water treatment plant, the Robert O. Pickard Environmental Centre, is set by the Ontario Ministry of the Environment at 1 milligram per litre, or one part per million.
In contrast, the Rideau River has very little assimilation capacity. It's relatively small and already degraded by nutrients coming primarily from agricultural activity and urban stormwater runoff. The city operates a small pilot plant in the village of Manotick that discharges into the Rideau River. Its effluent limit for phosphorus is set at 0.03 milligrams per litre, only 3% of the concentration that can be discharged into the Ottawa River. This kind of treatment is both difficult to achieve and very expensive.
Municipal waste water typically contains between 4 and 16 milligrams per litre of various phosphate compounds, in both dissolved and solid forms. In Ottawa, it's about 5 milligrams per litre, which does not sound like much, but it translates into about 750 metric tonnes per year.
Now modern secondary waste water treatment plants, such as Ottawa's, have very little difficulty achieving the 1 milligram per litre discharge target. An important point is that if you have modern sewage treatment, the technology is there, it's proven, and you can stay within those kinds of limits. To get down to the very low limit, which I was speaking about before, is problematic, and it's probably right on the cutting edge.
Phosphorus is removed from waste water in three ways. First, in the primary treatment process, the waste water is slowed down by passing through large tanks to allow heavier solid material to settle out. Biological removal and chemical precipitation occur in the secondary treatment process. In the case of the Pickard centre, this is called the activated sludge process.
Naturally occurring bacteria are used to absorb organic material, including dissolved phosphorus and iron or aluminum salt. In our case, ferrous chloride is added to convert dissolved phosphorus into a solid form that will precipitate out of the water. After being aerated to encourage bacterial growth, the mixture is allowed to settle out in large clarifiers, and the clean water is removed from the surface and discharged into the river.
The settled sludges are removed, returned to the beginning of the secondary treatment process, and added to the incoming waste water. It's important to maintain the correct balance between the amount of return sludge and incoming waste water. So excess material is removed to maintain the balance.
The waste material removed in the primary and secondary treatment process is pumped into large enclosed vessels known as anaerobic digesters, where different types of bacteria break down the organic material to produce water, carbon dioxide, and methane gas.
In Ottawa's case, the gas is removed and used in a cogeneration plant to produce electricity and hot water for plant processes and building heating. This saves the city about $1.4 million net in electricity and natural gas purchases.
The stabilized digested sludge, commonly referred to as biosolids, are then dewatered in centrifuges, much like the spin cycle of a dryer. The biosolids are about 33% solid and have the consistency of wet soil.
Ottawa's biosolids are beneficially recycled, either as a supplement in the manufacture of compost or directly by land application. In both cases, the phosphorus in the biosolids is available as a nutrient. This is a fairly common practice across the country.
As I mentioned previously, stormwater runoff also contains phosphorus from animal feces and fertilizer. In new urban developments, stormwater management ponds are used both to hold back storm flows to prevent erosion of downstream creeks and rivers and to provide passive treatment of organic waste and bacteria. Heavier materials settle out, and the action of plants and bacteria, including algae, remove organic materials and nutrients such as phosphorus. These ponds are capable of removing up to 95% of the incoming phosphorus.
Some of the sequestered phosphorus is eventually released when the plant life dies off in the fall. This is not problematic since the receiving water is too cold to support algal blooms.
That concludes my presentation.