Clearford’s Design Approach
Small Bore Sewers—A Proven Technology
Small bore sewers are a well-known type of sewage collection system employed around the world for efficient wastewater servicing of communities. The technology was first postulated as early as 1935; however, the first recorded installations were implemented in the 1960s in Australia, Africa and the US.
The technology is well documented in the United Nations Development Programme and World Bank publication “The Design of Small Bore Sewer Systems” (1985). Its success has led to adoption of small bore sewer design standards in many countries, including in the US (Environmental Protection Agency, Water Environment Federation), Australia and New Zealand (Australian/New Zealand Standard National Plumbing and Drainage Code), Canada (Ontario Ministry of the Environment), and India (Central Public Health and Environmental Engineering Organisation).
Making Gravity Work for You
Clearford’s approach is to use passive gravity-driven design throughout the Clearford One™ system to perform the critical functions of sewage conveyance and treatment. Harnessing gravity reduces reliance on power supply, which may be expensive or unreliable, and increases the sustainability of sewage servicing over the long-term.
Design of the ClearDigest™ smart digesters builds on the long-established performance of traditional septic tanks, which use gravity for the separation of solids and liquids in sewage. Heavier solids sink to the bottom of the tank while lighter oils and grease float to the surface, leaving substantially liquid wastewater in the middle clear zone of the tank that can be released from the tank to the sewer collection network.
The ClearConvey™ pipe network transports the liquid wastewater using gravity flow through a series of small bore sewers. Flow is more efficient because the sewage solids have already been removed from the wastewater, allowing for sewer design with fewer constraints. Whereas conventional sewers must achieve a certain flow velocity in each pipe in order to keep solids in suspension, ClearConvey™ can be designed with nearly flat pipe slopes without concern for sewage blockages. Small-bore-sewer pipes are optimized for flow conveyance rather than solids carrying capacity.
This is especially important in communities that use less water. For conventional systems, low water usage may not generate enough flow velocity to move sewage solids through the pipe network, resulting in blockages that are difficult and costly to remove from underground sewers. ClearDigest™ protects the ClearConvey™ pipe network from blockages, regardless of the sewage volume generated from each building. Clearford One™ operates continuously with any range of daily water usage, dramatically reducing the cost of operations, and enabling sanitation in communities where conventional whole sewage systems can’t be contemplated.
Harnessing Natural Processes
The design of the Clearford One™ system relies on naturally occurring processes for sewage treatment. ClearDigest™ smart digesters are configured to enhance biological treatment by anaerobic bacteria—the same abundant organisms that humans use to digest food in our bodies. These bacteria break down the pollutants in sewage and decompose the biosolids.
The bacteria become more efficient as they mature, so the ClearDigest™ design provides an optimum retention time and contact volume to maximize decomposition of sewage in the smart digester. A proprietary inlet device uses the kinetic energy of the incoming sewage to gently mix inside the tank to increase contact between the sewage and bacteria to enhance digestion.
The impact of ClearDigest™ is that 75% of suspended solids and 65% of biochemical oxygen demand (BOD) are removed from raw sewage in the smart digester tanks. This constitutes much of the secondary (biological) treatment that is typically required for wastewater treatment. Therefore, the pollutant loading to the ClearRecover™ treatment plant is significantly reduced such that the capital and operating costs are also reduced.
Modulating Flow at the Source
ClearDigest™ smart digesters modulate the sewage flow quantity and quality being delivered to the ClearConvey™ and ClearRecover™ components. This is achieved through ClearDigest™ design features that passively manage the fluctuation of sewage through the tank.
Daily peak sewage volumes are controlled through a patented outlet device in each smart digester that restricts the outflow from the tank. Clearford One™ system peak flows are substantially reduced compared to peak inflow volumes, lessening the need to oversize downstream collection and treatment infrastructure by as much as 50% compared to conventional sewage systems. The result is smaller pumps, smaller tankage, and smaller footprint for sanitation infrastructure.
New sewage is continuously mixed with older wastewater in the smart digester tank, creating a consistent pollutant loading that leaves the tank. The normalization of wastewater quality eliminates shock loading to the treatment plant, which allows the final optimized treatment package to target the specific effluent character for efficient treatment.
Optimizing the Infrastructure
By removing solids, reducing peak flow, and eliminating extraneous flows, Clearford One™ follows a different approach for optimizing sewage infrastructure using sealed small diameter sewer pipes with fewer and smaller system access points to convey liquid effluent for final treatment.
Conventional sewers use large pipes and manholes to accommodate sewage solids, peak flow, inflow and infiltration. However, these rigid components are increasingly susceptible to leaks through joints and cracks over the life of the system. Inflow of storm runoff and infiltration of groundwater use up the conveyance capacity of the system and add significant pumping and treatment costs for downstream infrastructure. Leaky joints may also allow sewage to pollute valuable soil and groundwater resources.
ClearConvey™ uses flexible high-density polyethylene (HDPE) pipes with thermally fused joints and watertight system access points that eliminate groundwater infiltration and sewage leakage to the environment. HDPE pipe is the same material relied upon worldwide in the natural gas industry for providing reliable fully-sealed pipe networks. Robust HDPE piping has a design life over 90 years. It is resistant to corrosion and biological attack, and is non-reactive in the environment.
Sealed sewers guarantee that only the sewage from the connected buildings is conveyed to the downstream pumping and treatment facilities. Groundwater and seasonal storm influences are eliminated from the design of the Clearford One™ collection and treatment system.
System Access Points
In conventional sewer systems, bulky manholes are typically required at every pipe intersection and change in pipe direction, slope and sewer size. These manholes are costly to install and become a significant source of inflow and infiltration over the life of the sewer system. Although intended for maintenance access to the underground system, they are often unsafe for workers because of the buildup of harmful gases, confined space entry, and corrosion of manhole components.
The Clearford One™ system does not require large manholes for maintenance access to the underground system. Instead, system access points (SAPs) are a safe no-entry alternative providing ground-level access for inspection and maintenance of the ClearConvey™ small-bore-sewer network.
SAPs are vertical clean-out structures spaced throughout the ClearConvey™ network at regular 80250 metre intervals. They are not required at pipe intersections and changes in direction or pipe size. This is because flexible HDPE piping is used with fused joints and fittings, allowing for smooth curves and pipe transitions throughout the small-bore-sewer network rather than rigid intersections. Furthermore, only liquid wastewater is conveyed in the system so there is no risk of blockages from sewage solids at these transitions in the pipe network.
SAPs are small-diameter sealed cleanouts that are typically fewer, shallower and easier to install than large conventional manhole structures. They are watertight while still allowing for venting from the small-bore-sewer network to prevent the buildup of gases and hydraulic locks in the system. They are configured to provide ground-level access for maintenance, including occasional flushing of the pipe network using a simple water hose and inspection using standard closed-circuit television (CCTV) camera technology.
When sewage pumping is required, Clearford One™ may use only a third of the pump stations required in a conventional system. This is because ClearConvey™ small bore sewers can have half the slope of conventional sewers, resulting in shallower pipes over longer distances. Furthermore, pumping is simplified by the use of smaller pumps that are only required to handle liquid effluent, not messy sewage solids. This reduces energy use, as well as capital and maintenance requirements.
Optimized Treatment Plant
ClearRecover™ treatment operations are simplified, taking up less space and delivering cost savings. Conventional treatment plants require large surge and settling tanks that take up significant real estate and add operating costs. Since ClearDigest™ tanks perform so much of the primary (physical) and secondary (biological) treatment, loading to the ClearRecover™ plant is significantly reduced—resulting in lower capital, operating and maintenance costs. At half the size of conventional treatment plants, ClearRecover™ can more economically employ the best treatment technology to meet the local clean water requirements, whether it is for reuse or release back to the environment.
Clearford One™—A Sustainable Solution
Clearford’s approach to optimizing the design and installation of sewage infrastructure for long-term sustainability enables responsible development and prosperity for communities. Clearford One™ is more efficient, longer lasting and less expensive than conventional systems.
The efficiency of Clearford One™ is driven by clear understanding of hydraulic principles and natural processes for wastewater conveyance and treatment. By harnessing naturally-occurring physical and biological processes like gravity and anaerobic digestion, Clearford systems are designed for low maintenance, reliability and minimal power consumption.
The components of Clearford One™ are selected for their constructability and lifecycle cost. By using high quality and locally available materials and construction methods, Clearford One™ systems are installed to last underground for over 90 years—almost twice as long as conventional sewer systems. That means maintenance disruptions and renewal costs are significantly less for communities.
Clearford One™ is less expensive to install than conventional collection and treatment systems because of the advantages created by our unique design approach. By removing solids at the source, modulating peak flow, and eliminating extraneous flows, sewage infrastructure is smaller, shallower and easier to construct. But the largest savings will come through lower lifecycle operating and maintenance costs.