WHAT IS WASTEWATER?
Wastewater is liquid waste. It is animal, vegetable, mineral or chemical matter
in solution or in suspension that residents and businesses flush down their
toilets and pour down their sinks and drains.
Wastewater drains into a network of pipes maintained by sewer serviced municipalities and the Regional
District of Nanaimo. Sewer systems are built to follow the natural slope of land, generally flowing towards the
sea front. This design allows gravity to do most of the work transporting the wastewater to one of four
wastewater treatment plants. For residential areas that are lower than adjacent lands or treatment plants,
the wastewater must pass through a pumping station to pump the liquid into the plants. Treatment of our
wastewater is an essential process that prevents contamination and destruction of our waterways, and our
natural water resources.
HOW IS WASTEWATER TREATED IN THE RDN?
Generally wastewater treatment involves the following processes:
Preliminary treatment - The velocity of the wastewater from the sewer lines is reduced as it enters the
treatment plant. This allows sand, gravel and other heavy materials to settle out into grit tanks. Mechanical
cleaning bar screens then remove rags, sticks, plastic and other foreign objects from the wastewater; this
part of the treatment plant is called the headworks. Bar screens may be used before or after the grit tanks.
All removed material collected by grit tanks and bar screens is washed and then taken to a sanitary landfill for
Primary treatment - Primary treatment allows for the physical separation of solids and grease from the
wastewater, and removes between 30-40 percent of Biological Oxygen Demand and 50 per cent of Total
The screened wastewater flows into a primary settling tank where it is held for several hours allowing
solid particles to settle to the bottom of the tank. Fats, oil and grease (FOG) are skimmed from the
tanks, dried and sent to the landfill. The settled particles are known as primary sludge, which is
collected and pumped to large digestion or holding tanks for further treatment and solids processing.
Presently, Greater Nanaimo and Nanoose Wastewater Treatment Plants provide primary treatment and
discharge treated effluent to the ocean.
Secondary treatment - Secondary treatment is a biological treatment process that removes up to 90 percent
of BOD and TSS. Following primary treatment, effluent is pumped to the secondary treatment stage. It may
take one of several forms; for example, either a trickling filter or a sequencing batch reactor. The growth of
microorgisms results from the consumption of organic matter in the wastewater as their food supply.
The micro-organisms create a solid organic material (sludge), which just like the sludge from primary
treatment. Secondary sludge is thickened and pumped to digesters for processing and solids
The French Creek and Duke Point Treatment Plants discharge secondary treated effluent into the
Final treatment - The wastewater that remains can be disinfected to kill harmful micro-organisms before
being released into receiving waters. Although there are many methods available to kill these micro-
organisms, ultraviolet disinfection is the method used at Duke Point Pollution Control Centre. This is the only
treatment plant in the RDN with this technology. At this stage, the treated wastewater, now called final
effluent, is discharged into the marine environment.
Solids processing - Primary solids from the primary settling tank and secondary solids from the clarifier are
sent to digesters for solids processing. During this process, micro-organisms use the organic material present
in the solids as a food source and convert it to by-products such as methane gas and water. Digestion results
in a 90% reduction of pathogens and the production of a wet soil-like material called "biosolids" that contain
95-97% water. In order to remove some of this water, mechanical equipment such as a belt filter press or
centrifuge are used to squeeze water from the biosolids, reducing its volume prior to being used in soil
amendment or composting.
BOD5 (Biochemical Oxygen Demand) is a measure of the quantity of oxygen consumed by microorganisms
to break down organic matter in water. A high BOD means that there will be less oxygen; and oxygen is
essential for the survival of aquatic life. Thus, high BOD levels result in the contamination of the receiving
TSS (Total Suspended Solids) are solid pollutants that would be captured on a fine filter paper. They are
visible in water and decrease water clarity. High concentrations of TSS can cause many problems for aquatic