January 21, 2020 by Biljana Ognenova
Wastewater plants are major electricity consumers. Applying intelligent electrical plant design principles could greatly reduce their power consumption and improve operational efficiency.
The focus of EEs should be on designing sustainable electrical network architecture as a major contributor to clean, green hydropower.
Water or hydropower is considered clean and renewable
energy, but this is not always so. Looking through a wider lens at its role in global sustainability—wastefully-managed water energy can downplay efforts made with other renewable energies in the electricity realm.
Wastewater management is, for example, an energy-intensive sector. Water and wastewater management in the U.S. only amounts to 5% of the overall energy consumption, while the same figure when measured worldwide approaches 3%.
Wastewater systems emit more than 45 million tonnes of greenhouse gases (GHGs) per year, including methane and nitrous oxide produced from purifying petroleum-based household and industrial products. The main problem is that fossil fuels account for a major part of the GHGs from wastewater plants, especially in oil-heavy industries, such as oil refineries.
According to Huber Technology, modern wastewater plants should consume between 20 and 45 kWh /(PE•a), [PE = Population Equivalent]. These are the numbers engineers should keep in mind in the design. The lower number in this range applies to large plants that serve >100,000 PE, while the higher number applies to smaller wastewater plants.
Apart from the plant size, designs that include nutrient removal and anaerobic digesters consume more energy.
There are many ways in which electrical and electronics engineers could improve the energy efficiency of wastewater facilities, many of which would focus on enhancing and optimising electrical processes by applying intelligent design principles.
A sewage plant in Switzerland.
Design standards and principles are strict and encompass applications on all levels of plant design—from equipment safety for high-voltage and low-voltage equipment to standardised quality assurance for equipment suppliers and installers. Such QA should include transformers, motors, variable, speed controllers, switchgear, switchboards, and instrumentation systems.
If we take the example of the Australian Water Corporation, the design process should take into consideration the scope of work, on-site environmental conditions, equipment location, the identification of hazardous areas, as well as factors that affect lighting, paging, communication, fire and security systems. Process control system architecture and hardware, data communication methods, and field instrument types must also be a part of the design document. Overall, wastewater treatment plants place serious demands on electrical design engineers.
Wastewater filtration goes through a four-stage process: primary treatment, secondary treatment, anaerobic digestion, and disinfection—including complex separation of solids from liquids and ammonia purification aided by bacteria. But that bacteria need oxygen, food, and temperature to thrive and do their job, and that oxygen is a major energy spender.
Curiously enough, wastewater plants create organic matter that could produce five times more energy than they spend, but managing all these aspects of the process is not easy. Generating electricity onsite and using predictive analytics platforms, such as the one created by HelioPower, can optimise plant energy efficiency by applying action immediately, particularly upon insights picked up in real time—especially useful in terms of reducing downtime.
Further in the interest of downtimes, it’s important to consider how motor management contributes to overall plant efficiency. Motors connect treatment facilities with the electrical network and consume most of the power load. Despite being relatively cheap, motor failure can cost the facility an arm and a leg. One way to reduce downtimes is to use intelligent low-voltage and medium-voltage motor starters and motor control systems, including improved motor protection and safety, as well as operator and technician safety.
An electrical network design for wastewater plants (T4), an open medium-voltage loop architecture recommended for large urban wastewater plants. Image Credit: Schneider Electric.
Conventional electrical design methods—that include a circuit breaker, a contactor, and a thermal relay scheme—work only for overloads and short circuits. These constitute only a third of all motor failures. One of the major operators in this area, Schneider Electric suggests going a step beyond conventional motor protection systems. Its concept is based on thermal overload protection.
This new system could protect against many external factors, as well as against abnormal power load or supply, insulation failure, or incorrect wiring. People and equipment safety and minimal downtimes are prioritised accordingly.
Size seems to be a very important issue in plant efficiency. Therefore, Schneider recommends various electrical network architectures according to size, and in keeping with this, classifies wastewater plants into four groups:
Small autonomous plants (T1): 1,000 – 5,000 m3/day, 1,000 – 10,000 inhabitants, 25 – 125 kVA power demand
Medium-sized plants (T2): 5,000 – 50,000 m3/day, 10,000 – 100,000 inhabitants, 125 – 1250 kVA power demand
Large plants (T3): 50,000 – 200,000 m3/day, 100,000 – 500,000 inhabitants, 1.25 – 5 mVA power demand, and
Large to very large plants (T4): 200,000 – 1,000,000 m3/day, 500,000 – 1,000,000 inhabitants, 5 – 25 mVA power demand.
The goal is to work on meeting stringent regulatory requirements for water conservation by increasing the wastewater plant scalability and thus, its lifetime.
Note that recommendations for electrical power monitoring differ according to size, too: as the size of the plant grows, so does the complexity of the network architecture.
Small plants should include a radial single feeder configuration with low-voltage power supply that supplies all units from a single low-voltage switchboard and conventional motor control systems.
Medium-sized plants incorporate a medium-voltage supply as a part of the electrical network, separate low-voltage switchboards, and more motors for greater harmonics and degrading power factor.
A diagram that displays how process, power, and energy-efficiency data is collected by intelligent electrical devices and sent to various information systems. Image Credit: Schneider Electric.
There are two choices for large and very large plants:
Double radial architecture—a simple and cost-effective solution with low power availability and challenging site extension; and
Open medium-voltage loop, which is more suitable as the size of the plant grows and requires a large installed power base because it restricts additional infrastructure or cabling. Open-loop and can be easily extended by redesigning the architecture by cutting the loop and adding new power loads.
Very large plants have many factors that affect their efficiency. They must filtrate a large volume of water. The medium-voltage network must have high redundancy levels, including multiple motors with variable speed drives that require careful management and monitoring.
A double supply system is recommended to power the plant in the event of a malfunction. Which architecture variant will be implemented in a specific area depends on the local customs and regulations, as well as the site layout.
But there are certain automation benefits for the open-loop architecture that should be considered: automation provides faster and safer operability, increases personnel safety, and takes into account all network configurations and limited downtimes. The overall network efficiency also depends on having an efficient metering architecture. Such architecture is useful for both power monitoring and control, as well as energy information management.
These are just a few of the recommendations for designing intelligent wastewater plants. The larger the plant, the more important its ‘intelligence’ is. And here, ‘intelligence’, could be applied literally, in the sense of using predictive analytics that provide insights on the basis of data supplied through the process of water filtration and the direct reduction of GHGs.
It could also be applied figuratively: when electrical networks are designed with size in mind—namely to extract the optimal operational capacity at lower costs, less downtime, and improved site extendability.
