Evolution of lagoon effluent enhancement to meet stringent effluent criteria

By R. Willcocks, P. Eng., Triton Engineering Services Ltd.

Aerial view of Dundalk lagoon system following 1984 expansion. Photo – Bob Willcocks

Articles in ES&E magazine as well as papers at technical conferences have provided an insight into the use of open air intermittent sand filters to enhance effluent quality from facultative lagoons. In Ontario, this is known generically as the New Hamburg process, as it was first applied in the early 1980s in New Hamburg in the Region of Waterloo, Ontario. It is known from papers presented that similar technology has been applied in other jurisdictions.

Effluents from facultative lagoon cells can have problems with elevated ammonia and hydrogen sulphide (H₂S) levels in the spring, after the ice comes off, until aerobic conditions return. In the summer, algae blooms elevate suspended solids levels in the effluent and, as blooms die off in the late Fall, there can be an increase in BOD levels. Both conditions make the effluent unacceptable under the Ontario surface water quality objectives. The addition of the intermittent sand filter as an add-on effluent treatment was designed to address these issues. The intermittent sand filter exposes lagoon effluent to open air to provide oxygen and to coarse sand media to provide a growth site for bacteria which promotes the conversion of ammonia in the effluent to nitrate through nitrification. Exposure to oxygen also removes the H₂S. The media acts as a physical barrier to the migration of effluent solids into the final effluent.

The Ontario Ministry of the Environment has accepted the process as a lagoon enhancement which allows for continuous discharge of effluent which has been accepted as Best Available Technology (BAT). With this enhancement, some projects allow for continuous effluent discharge for an extended period instead of the twice yearly seasonal discharge which requires a 180-day retention period. The limiting factor to the process was established as the need to retain effluent for the coldest winter months when the nitrification treatment could not be maintained because ambient liquid temperatures fall to less than 4°C. This is the lowest viable temperature for maintaining nitrification, established in pilot testing at McMaster University. Depending upon the location in Ontario, the original treatment and storage requirement of 180 days has been reduced to a three or four month storage period, effectively increasing the ‘capacity’ of the same lagoon volume by 33 – 50%.

This article discusses a project designed to be constructed during 2000 to provide a different form of filtration system which can allow for winter discharge of lagoon effluent, potentially shortening the retention period to as little as two months while allowing for continuous discharge for the balance of the year, if necessary. The limitation to two months may not apply in cases where temperatures are more moderate. In this particular case, an effluent total ammonia criterion which requires nitrification has not been applied by the Ministry.

As is discussed below, the primary focus for effluent improvement is phosphorous along with overall reductions in BOD and TSS. The new criterion accepts unionized ammonia as the winter ammonia requirement, while discharge in the summer regulates the total ammonia which also meets the unionized requirement.

Background

In 1984, Triton Engineering designed a ‘post-aeration’ lagoon system for the Village of Dundalk which is located some 125-km northwest of Toronto. The facultative lagoon system was also expanded to a four-cell system to provide 180 days of retention at an increased design flow. There were allowances made in the lagoon volume calculations for sludge retention and to allow for a carryover volume. Alum is added to the raw wastewater at the pumping station for phosphorous treatment. The lagoons generally operate in a four-cell series pattern although cell number three acts as a bypass receptor for the pumping station, so it receives raw wastewater during wet weather conditions or during power failures. The sewer system is prone to elevated I/I flows leading to relatively weak wastewater during wetter periods.

As far as is known, the post-aeration system in Dundalk was unique in Ontario and not, perhaps, replicated at any other location.

The Ministry had concerns related to elevated levels of hydrogen sulphide which could be discharged from a lagoon during winter discharge. This system was installed before the New Hamburg system was accepted as a standard design approach. In a New Hamburg system there is no discharge during the coldest months and when there is, the exposure to oxygen also treats the hydrogen sulphide. As lagoons typically nitrify throughout the summer, the major proportion of the lagoon effluent which is discharged during the Fall is nitrified. Discharge of winter effluent can reflect increased levels of ammonia and, as both ammonia and hydrogen sulphide are toxic to aquatic organisms, there is a need to treat for these or to prevent them from forming.

In Dundalk, ammonia was not considered to be a primary concern because there is an intermittent flow municipal drain which acts as the mixing zone before effluent reaches the Grand River. The 1984 work focussed on the removal of hydrogen sulphide, although there was some discussion whether an elevated dissolved oxygen level in the effluent would also serve to strip ammonia as well. Since the lagoons were expanded and the post aeration cell was installed, the system has been generally drained over the entire winter and fill ing of the lagoons starts in the spring. The effluent has been generally well within the 1984 Approval parameters.

System operating problems arose as the result of increased flows during summer and Fall wet years. There have been occasions when the lagoons are full before the prescribed start of the discharge period. The Ministry directed that the municipality evaluate alternative means to increase the lagoon capacity. During dry years, there is an abundance of storage capacity.

Process Selection and Effluent

Requirements The Class EA for the project reviewed available treatment options and the selected preferred technical alternative was the addition of a post lagoon filter to improve effluent quality and to increase overall capacity without increasing lagoon volume. The limitations of the New Hamburg process with respect to minimum operating temperatures did not permit a sufficiently large capacity increase due to ambient winter temperatures in Dundalk. To allow for increased capacity, the lagoon system has to provide an effluent which can be discharged during the summer. The design allows for a total ammonia criterion but effluent phosphorous levels became the primary constituent of concern. Because lagoons nitrify during the summer through natural processes, the new treatment system did not require an open air filter to provide nitrification.

The Ministry’s Southwest Region provided new effluent criteria which saw a general lowering of the effluent parameters of BOD, SS and phosphorous levels. With respect to effluent ammonia concentrations, discharges in the winter have to meet the unionized ammonia criterion and not a total ammonia criterion which is an important difference from the typical approach. A continuous discharge lagoon in Niagara-on-the-Lake is the only other project which has a similar effluent criterion as far as we know. The effluent criteria also requires that an objective of 4 mgl of dissolved oxygen (DO) be maintained in the discharge. This is provided by the post-aeration cell.

The effluent criteria can be rationalized because the receiving watercourse is an intermittent flowing municipal drain which discharges into the upper Grand River some distance away. The drain is considered to be a mixing zone with limited natural flow, essentially being a dry ditch for much of the year. This Region of the Ministry has accepted the dry ditch effluent discharge criteria for a number of years. When effluent is being discharged, which is non-toxic to organic life in the stream, there is a perceived improvement in quality within the stream or ditch. Flow sustains aquatic life which would otherwise disappear as the stream dries up from natural runoff.

The background of applying the unionized component of ammonia in the winter relates directly to the toxicity issue of ammonia and not to the combined effects of oxygen demand and toxicity. Under winter temperature and pH conditions, meeting of the unionized criteria can allow for total ammonia concentrations to be as high as 16 mgl. In a large receiving body such as Lake Ontario or in the case of the dry ditch receiver, the potential for toxicity to affect marine life is substantially reduced, especially during the winter. In Niagara-on-the-Lake, the upper limit was established at 20 mgl, at which point ammonia treatment is brought on line to lower the ammonia level. In that particular installation a physical-chemical treatment process is used for treatment as opposed to filtration. Chemical addition precipitates the phosphorous which collects in a clarifier.

The Dundalk design allows for continuous discharge at times of the year when it is necessary to lower levels in the lagoon. The de.sign also allows for an increase in the design capacity without an increase in lagoon volume. Modelling of the system considered the volumetric storage requirements during a wet year such as occurred during 1996. By allowing for a specific rate of discharge based on the filter capacity, net fill volumes were calculated per month. It was necessary to size the effluent treatment system to ensure that the lagoon volume storage requirements were not exceeded while still considering that discharge during the coldest months may not be possible. In most years, there will not be a need to strictly control the system although there will be always a need to maintain an adequate buffer. In the design year condition, it has been assumed that there will be two months when discharge is not necessary.

Description of Project

Dundalk has the claim of being the highest municipality above sea level in Southern Ontario, and, being south of Owen Sound, is squarely within the snowbelt region. Accordingly it became essential to consider a filtration system which could be housed in a building.

The project includes a modified dual train package water treatment filtration unit (manufactured by Napier Reid) following the aeration cell which is housed in a building. The selection of this type of filter was based on the need to consider the impacts of such things as frazzle ice which can develop in water which is at or slightly below freezing level but which does not freeze because of kinetic energy. Some heat is added by aeration using diffused air and, as the filter will be housed in a building, further loss of heat is limited. The process will operate on batch cycles and with mechanical pump backwashing, there is more potential to prevent and/or remove frazzle ice. The basic safeguard against freezing is that it will not be necessary to operate the filter under the coldest temperature conditions.

The filter module includes similar components to the typical water filtration unit. This includes mechanical pumping for backwashing, an air scour for media surface cleaning to remove algae, and separate backwash handling equipment. Filter backwash water is pumped to the lagoons for retreatment and the source of the backwash water is stored effluent following the filter. In cases of very large algae blooms, there can be algaecide added to the influent pumping chamber or even the aeration cell.

The focus of this treatment process is a lower effluent phosphorous level. To allow for lower effluent phosphorous levels, chemical is added before filtration in the form of alum or alum supplemented with polymer addition during colder temperatures.

Potential Benefits of Process

As noted, the effluent parameter of most concern is the phosphorous which requires low effluent suspended solids levels to ensure compliance with concentrations of 0.3 mgl or less. While ammonia is generally regulated between 1 and 3 mgl for design objectives in the summer and winter respectively, there can be a case made to focus on the unionized ammonia criteria in the winter. With the presence of the post aeration cell, there is a demonstrated control provided for effluent hydrogen sulphide levels.

The Ministry’s Southwest Region has been requiring elevated Dissolved Oxygen (DO) levels in the effluents from treatment works to mitigate oxygen demand on the receiver which may have a very low natural DO level. A facility operator at a plant which has this same effluent criteria commented that the best place to catch fish on the Thames River in the middle of the summer is around the outfalls from the three plants which he supervises. In this Region, the New Hamburg systems which are installed have also been required to provide aeration to increase effluent DO levels.

The major benefit of this approach over New Hamburg systems is the continued operation for longer periods in the winter. While the water filter provides limited potential for nitrification on the media, the lowering of effluent suspended solids levels and phosphorous concentrations may be the more appropriate approach in some cases if winter ammonia discharges are considered in terms of the unionized component only. This would compare to the fact that the New Hamburg filter process does not specifically treat for phosphorous levels and relies on seasonal or continual chemical addition to the lagoon to allow for this.