By Dick Menon
In Manitoba, the most common form of wastewater treatment are facultative lagoons. They are less expensive than other types of treatment systems and require little hands-on operation and maintenance. Manitoba is known for its warm sunny summers and lagoons are able to use natural sunlight and oxygen, to breakdown organic matter in the waste stream.
The popularity of lagoons is not only due to lower capital and operating costs, but also to the fact that, in many locations, a continuous effluent discharge is not possible. For a stream or a river to accept continuous discharge, a minimum flow is required. In Manitoba, with a few exceptions, such as the Red, Assiniboine, Souris and Winnipeg Rivers, most streams do not have the minimum winter flows to allow for a continuous discharge from a wastewater treatment facility. This means that treatment systems in such areas need lagoons for winter storage.
Many lagoons in Manitoba were built in the 1960s and 1970s, which means that they have reached their design capacity and need to be expanded. Since they are built in modules (cells), it is easy to add more as the community grows.
However, the landscape has changed over the last 40 years and many municipalities have to develop and implement nutrient reduction plans for their wastewater treatment systems. For cities, with an increasing tax base and steady growth, system expansion is easier to accomplish than it is for small towns, villages and hamlets.
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There are at least three issues that small municipalities need to consider, when faced with having to expand their lagoons.
In Manitoba, the most common mechanism for undertaking capital works is through borrowing, i.e., debentures. Municipalities need approval for their “borrowing by-law” from the Manitoba Municipal Board. This is a quasi-judicial provincial entity that reviews and approves, amongst other things, all borrowing by-laws. It looks at the financial capability of the municipality to incur capital debts.
A simple rule-of-thumb is that 7% of municipal assessment, less capital debt, is a measure of how much additional capital debt a municipality can get into. One with an assessment of $100 million and a capital debt of $4 million has $3 million borrowing capacity available for further works. If the same municipality is now faced with upgrades to their municipal wastewater treatment system, estimated to cost $5 million, what options do they have?
Municipalities can apply for grants from higher levels of government, but demand for grant funding in many cases is 50 times more than the available funding. If they are not successful in receiving grants, then the only option available is from municipal ratepayers. Generally, capital debts/ debentures are repaid over a 20-year period. Therefore, if a municipality has $2 million of its $4 million debt retiring within two to five years, it can plan to have new debt incurred to match the debt retirement.
Sewer rates and reserve funds
Another way to stretch municipal budgets for capital works is to plan for it through sewer rates. This is becoming a more attractive mechanism, but requires proper planning. In the past, utility rates have included minor capital works, but, over the last 10 years, major works are being included in utility rate studies. Since this is a “user pay” method, it is more attractive than using an assessment based repayment method, which may exclude many properties.
Extending life spans of municipal lagoons
Municipal lagoons require very little hands-on maintenance. In many instances, the adage “out of sight, out of mind” fits lagoon operations. This is until issues arise, such as meeting new regulations or loss of capacity. Meeting new regulations can be achieved with the addition of tertiary treatment, such as a wetland.
When it comes to capacity issues, sludge accumulation in the primary cell is often ignored. Many lagoons also accept trucked-in waste, so it is not uncommon to anticipate sludge accumulation of 200 m3 per year. Over time, accumulated sludge can reduce the hydraulic capacity of the lagoon. Municipalities often look at adding an additional cell, or removing accumulated sludge as options for regaining lost capacity. However, these options can be expensive.
A relatively less expensive option is the use of bacteria to break down accumulated organic sludge. Nordevco Associates Ltd. market a product called BactiDomus®, which reduces organic sludge in lagoons. It uses a porous and non-toxic inorganic carrier material to deliver bacteria into lagoon sludge. Once there, the embedded bacteria produce enzymes that break down the organic sludge to its basic constituents (water, CO2, nitrogen gas). Case studies have shown that, if applied in early spring, it can reduce accumulated sludge by over 70% in volume.
BactiDomus comes in a granular form and is easy to apply, with two people in a boat, over the surface area of a primary cell. It is supplied in 25 kg bags and does not require the use of any heavy equipment.
A sludge profile is taken before and after application to estimate the amount of the material required. The accompanying graphs from a case study indicate the effectiveness of using bacteria to reduce accumulated sludge and regain lost lagoon capacity.
When it comes to stretching municipal infrastructure budgets, bacterial applications can enhance the efficiency and extend the life of sewage lagoons. This gives municipalities the time to efficiently plan for expansions.
Dick Menon, M.Eng., P.Eng., FEC, works with Nordevco Associates Inc. This article appears in ES&E April 2016 issue.