SEWER MAINTENANCE OPTIMISATION SYSTEM
Most collection system sewer overflows are a result of capacity
problems. When large rain events occur and overflows happen,
the newspaper headlines will often read: “recent rains cause
sewer overflows”. The reality is water entering the sewer system
from inflow and infiltration (“I&I”) overwhelms the designed
carrying capacity of the pipes - not designed to handle unexpected
volumes of rain water - in the collection system, leading to spills. Detecting and monitoring for I&I is covered in a separate note under “Inflow and Infiltration Detection and Monitoring” on the SWAN site. The second capacity issue facing collection system operators is decreased capacity of pipes due to buildup of grit, debris, fats, oils and grease (FOG), and tree root growth.
The common practice to minimise spills due to buildup is to clean pipes frequently enough to clear out the buildup with high pressure water or in some cases, pipe-damaging tools like root cutters. This technique has been proven to be highly effective. For example, the City of San Diego, CA (USA) reduced their sewer overflows by 10X over the past 10 years using this method of high frequency cleaning.
Informed HFC also monitors what materials are collected by the cleaning apparatus every time a cleaning is done to re-assess the frequency of cleaning. For example, if a monthly scheduled pipe is visited three months consecutively and no debris or roots or FOG are collected, the cleaning frequency may be dropped to quarterly. Conversely, if an overflow occurs at a site with a given cleaning frequency, the cleaning frequency would be increased to avoid a spill in the future.
A prudent collection system manager tends to over-clean by definition – the more you clean, the cleaner the pipes, the lower the chance for a capacity-limited spill occurring, wet or dry weather. Despite the successes of this method, there are a two serious weaknesses that can be ameliorated with the application of available technology.
Weakness #1: a prudent and conservative collection system manager will over-clean their collection system to minimise risk. This is because the manager does not have real-time capability of assessing conditions in various locations in their collection system. Over-cleaning is costly, wastes valuable resources and labor, adds operational risk, and contributes to carbon footprint.
Weakness #2: a collection system manager only gets data on the pipeline condition when they physically visit the site. Between cleanings, or between times when a video camera is used to observe the site, the manager is effectively blind to the conditions at the pipe. Adjusting the planned cleaning schedule is a guessing game based on sparse data.
The function of a data-driven cleaning in collection systems is to relieve these weaknesses by monitoring - continuously and in real-time zones in a collection system designated high risk by the manager and thereby worthy of HFC.
A sewer maintenance optimisation system consists of the following components:
Data collection and analysis
Decision support for cleaning orders
Sewer collection systems generate a signature associated with daily, weekly, and seasonal usage and the goal of optimised cleaning is to compare real-time signatures and trends that deviate from this baseline signature, effectively providing remote condition assessment between cleanings. If there is no capacity constraint caused by any kind of blockage, there is no need to clean the pipe. If a blockage is beginning to form, the water level trends will betray this and it’s time to schedule a cleaning, particularly if a storm is on the way.
Sensors for measuring water level can be water level sensors or flow meters. Water level sensors are preferred due to their cost, stand-off measurement, and ease of installation and service. Flow meters may be used, but flow itself provides no distinct advantage in this application.
Communications to and from the remote sensors can take place via a variety of means:
Local wireless, such as Bluetooth, WLAN (IEEE 802.11)
Wide area wireless, such as private radio networks
Commercial wireless, such as cell phone providers or satellite data providers
Two-way communication is generally preferred, as it allows control of remote sensors and remote operational parameter modification. Data collection requires that historical level/flow data is available and analysis is dependent on the ability to detect trends in water levels over days or weeks. The decision to clean a particular line segment or a group of contiguous segments is based in the existence of – and severity of – a trend in the water levels. Trends can take a variety of forms, and it may take several cleaning cycles to discern which trends are most important in terms of scheduling cleaning.
The benefits of using a continuous, sewer maintenance optimisation system include:
Lower operating costs: In some case studies for monthly cleanings, frequencies of cleaning were dropped by 80% - 90%, resulting in cost savings – net the costs of monitoring - of $1,500 to $4,000 per location per year;
Lower risk of spills: Since these high risk locations were also monitored 24/7/365, unusual level events that occur prior to spills could be detected and alarmed. In one case, spill risk has been reduced by 99%;
Lower capital costs: Cleaning of clean pipes with aggressive methods such as high pressure jets causes long term damage to pipes, decreasing lifetime. Lowered cleaning frequencies based on need will extend the lifetime of pipes;
Lower risk to personnel: Less time will be spent cleaning and time in traffic will be lowered;
Lower carbon footprint: Lower visits to high frequency translates directly into lower fuel usage;
Increased management efficiency.
The installation and operation of a sewer maintenance optimisation system requires the following elements:
A plan of deployment of sensors, based on the number and distribution of high frequency cleaning sites;
The physical deployment and calibration of the sensors;
Testing and verification of the communication system;
Methods of communication for the deployment of cleaning assets on an as-needed basis.