Wastewater Treatment

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Halifax Regional Municipality currently owns and operates a total of 12 sewage treatment plants, a landfill leachate facility, a domestic water plant, a sludge transport unit and a biosolids stabilization lagoon. The total process capacity of all HRM sewage treatment facilities is approximately 15 million gallons per day.

Operation and maintenance of the HRM facilities is performed by the Wastewater Treatment Division of Environmental Management Services. The HRM pollution control plants incorporate a large diversity of treatment technologies ranging from established methods such as extended aeration and trickling filters to hybrid anaerobic reactors, rotating biological contactors, pure oxygen systems and engineered marshland disposal. These facilities play an important role in protecting public health and the environment.

Why do we treat wastewater?

The most basic answer is to make dirty water clean. Treatment facilities simply compress the organic decomposition processes which take place in nature. This is performed by a combination of physical, biological, and chemical treatment stages. Nature (receiving waters) can only accept small amounts of sewage before becoming polluted, that is, natural bacteria feed on the sewage organics and create an abnormal amount of dissolved oxygen uptake. Dissolved oxygen which exists in minute amounts (10 parts per million @ 20ºC), is required by all marine life for survival. One of the principle objectives of wastewater treatment is to prevent as much of this "oxygen-demanding" organic material as possible from entering the receiving water.

Other objectives of wastewater treatment include, removal of objectionable items, nutrients and heavy metals.

The degree or level of treatment required often depends upon:

• Assimilation capacity of the receiving water.

Common terms used in wastewater treatment

Preliminary or pretreatment is the very first stage, it is the removal of larger materials and grit that if not removed could hinder subsequent treatment processes. It is accomplished through the use of equipment such as bar screens, macerators, comminutors, racks and grit removal systems.

Primary treatment is usually comprised of preliminary treatment followed by primary clarifiers which remove approximately 50% suspended solids and 35-40% B.O.D.5. This is accomplished by channeling flows through large tanks with residence times of 2-4 hours, thus allowing suspended solids to settle.Post disinfection and a biosolids treatment process are normally included. Enhanced Primary Treatment can be performed by the addition of a coagulant such as ferric chloride along with a polymer, improving the degree of S.S. and B.O.D. removal to 80% and 60% respectively.

Secondary treatment involves a primary process and a biological treatment stage. There are many biological processes in the treatment operation with the majority being variations of fixed film and mixed culture applications. The activated sludge process (mixed) is achieved by establishing large diversified cultures of bacteria. The bacteria metabolizes and provides the enzymatic breakdown of organic components, ie., liquids, carbohydrates, proteins and cellulose, in the wastewater. 90% removal rates of B.O.D. and suspended solids are typical of secondary treatment.

Tertiary treatment is comprised of a three stage process which further improves effluent quality past the primary and secondary treatment phases (see above examples). Usually 95-98% removal rates are achieved. Often, nutrient removal is included with both secondary and tertiary systems.

BOD5: Biochemical Oxygen Demand is a biological laboratory procedure that measures the rate of oxygen use while stabilizing decomposable organic mater under controlled conditions of time (5 days) and temperature (20ºC). It is an important regulatory parameter used to indicate organic strength of the wastewater. Typical BOD5 of raw domestic sewage:240 mg/L.

Suspended Solids: Particles in wastewater, usually 75% organic and averaging 200 mg/L in raw sewage.

Fecal Coliform: A type of coliform bacteria which originate from the feces of various warm blooded animals (humans inc.). Usually a strong indicator of pollution or possible pathogenic bacterial contamination.

Nutrients: Substances which are required to support living plants and organisms. An overabundance of phosphorus and nitrogen can stimulate excessive plant and algae growth under certain conditions. In fresh water, phosphorus is the limiting nutrient while nitrogen is the limiting nutrient in salt water.

Disinfection: The process designed to kill most microorganisms in wastewater, including pathogenic bacteria. There are several ways to disinfect, namely, chlorination, ultra violet light, ozone, chlorine dioxide, etc.

Operations

Laboratory Analyses

Various wastewater and water samples are collected on a routine basis for analysis in six onsite laboratories. Approved methods are used to determine physical, chemical, and biological characteristics of the samples. Data generated is used for process control, report generation, and to ensure that plants are meeting effluent restrictions as specified by the various regulatory agencies. The Mill Cove laboratory is certified by the Canadian Association of Environmental and Analytical Laboratories. A quality assurance/quality control program for all of our labs is administered by the Mill Cove Lab Analyst, this helps to ensure consistent and accurate results.

Training

The wastewater/water treatment industry employs a multitude of diverse technologies. Stringent regulatory guidelines increase the need for technologically advanced treatment facilities. As a result, constant training and upgrading is carried out, by both in-house and outside sources to ensure that personnel are able to keep pace with technology and operate HRM treatment facilities with maximum efficiency. All supervisory and technical staff are currently certified under the Nova Scotia Department of Environment and Labour mandatory certification program.

Safety

All supervisory and technical staff working in the HRM Wastewater Treatment (WWT) section share responsibility in providing a safe work environment, for themselves, fellow plant personnel, and for visitors to their facilities.Each treatment facility is equipped with all necessary safety equipment and supplies. Copies of the Occupational Health and Safety Act of Nova Scotia, The Occupational Safety General Regulations, HRM Policy and Procedures Manual, Personnel Protective Equipment Manual, Nova Scotia Occupational Health and Safety First Aid Regulations, as well as many other safety standards are on hand at each treatment facility. Union and non union employees are represented on both the Central Joint Occupational Health and Safety(JOHS) Committee and the Main JOHS committee. WWT staff are committed to ensuring a safe work environment and participate, on a regular basis, in training for: Confined Space Entry, First Aid/ CPR, Transportation of Dangerous Goods, Traffic Control Person, Traffic Control Signage, and WHMIS. As well, staff participate in other training such as "Due Diligence, Electrical Lockout/ Tagout, Excavation/ Trenching, Proper Use of a Chainsaw and Chlorine Handling & Safety. Safety Equipment such as gas detectors, lifting devices, fall arrest protection equipment, etc. is inspected and certified. "Walk abouts", and informal and formal workplace inspections are part of the daily routine. Special attention is given to some of the inherent dangers that may be present in the workplace, such as explosive gases, electrical hazards, exposure to infectious diseases, oxygen deprivation, confined space entry, exposure to chemicals, physical injuries etc."Safety" is, and must be, a part of the daily routine for HRM WWT. staff.

Managed Maintenance

As with any industry, unexpected equipment failures and shutdowns can be costly, with an adverse affect on the effluent quality of a facility. To avoid such situations HRM utilizes a comprehensive preventative/predictive maintenance program. This program is comprised of routine service schedules and computerized records using Computerized Managed Maintenance software. Vibration analysis technology utilizing a software interface assists staff in assessing potential equipment problems.

Supervisory Control and Data Acquisition

HRM treatment plants are all linked to a SCADA system. Analog and digital points at each plant are sent by radio telemetry to a central computer. Data from any one of our facilities can be remotely accessed from any PC equipped with a modem and a communications program. A password is required to gain access. The Mill Cove Facility uses a PLC (programmable logic controller) based system which allows remote monitoring and control in real time.

All plants are attended only during the daytime periods, the SCADA system and Mill Cove PLC allow process monitoring, process control and data acquisition during off hours. SCADA also allows long and short term trending of data which can be used to identify potential problems.