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Kristian Sahlstedt, Director, Wastewater Treatment, Helsingin seudun ympäristöpalvelut HSYFact box: HSY
• HSY (Helsinki Region Environmental Services) is a municipal federation, which provides water supply, sewerage, and waste management services for the Greater Helsinki region
• Population served appr. 1.1M
• Water supply and sewerage services funded 100 percent by customer fees
• 80 M m3/a of drinking water produced
• 130 M m3/a of wastewater treated
• Turnover ~350 MEUR, investments ~180 MEUR/a
HSY (Helsinki Region Environmental Services) is a municipal federation, which provides water supply, sewerage and waste management services for the Greater Helsinki region as well as information on the Helsinki metropolitan area and the environment.
Improvement of energy efficiency is a long-time strategic goal of HSY. By 2030, our entire services shall be carbon neutral, and we shall produce an amount of renewable energy equal to no less than 100 percent of our own consumption. In addition to creating a positive climate impact, energy efficiency reduces our dependency on liquid fuels and electricity bought from the market, thus improving both the crisis resilience and economical performance of our services. In this article, we present some key features and results of the energy-related actions implemented at our wastewater treatment plants (WWTPs).
“In addition to creating a positive climate impact, energy efficiency reduces our dependency on liquid fuels and electricity bought from the market, thus improving both the crisis resilience and economical performance of our services.”
Wastewater Treatment and Energy
About 60 percent of the energy consumed by HSY and 80 percent of the energy produced by HSY are attributed to wastewater treatment, making this service the key player in our energy balance. HSY owns two WWTPs, dimensioned for PE 1.5 M in total. Both plants are built inside bedrock and are based on biological-chemical removal of nitrogen and phosphorus by activated sludge and tertiary filtration. It include anaerobic digestion of biosolids, which produces biogas. Electricity is produced in biogas-powered combined heat and power (CHP) units and organic rankine cycle (ORC) exhaust gas turbines. In addition, electricity is produced by solar panels installed on top of the WWTPs and pumping stations. Heat is generated from CHPs, biogas boilers, and heat recovery from treatment processes and exhaust air. In addition, local energy companies utilize up to 125 MW of heat and cold recovered from treated wastewater for district heating and cooling; however, these activities are not included in HSY’s energy balance.
Energy Optimization at Viikinmäki WWTP
HSY’s Viikinmäki WWTP (PE 1 000 000), the largest one in the Nordic countries, is responsible for 40 percent and 65 percent of HSY water services’ electricity and heat consumption, respectively. A series of technical and operational improvements implemented during the latest 10 years have increased the WWTP’s electrical self-sufficiency from 50 percent in 2010 to 95 percent in the 2020s. Periodically, 100 percent self-sufficiency is exceeded. The end result is a system, where the efficiency of own energy production and consumption is maximized and, in addition, own production is balanced with energy purchase from the market according to hourly spot prices, resulting in optimisation of costs. Moreover, the WWTP is 100 percent self-sufficient in heat. The system is presented graphically in Figure 1 and described below.
Figure 1. Schematic representation of the energy and heat balance Viikinmäki – Vanhakaupunki. ORC = Organic Rankine Cycle, CHP = Combined Heat and Power plant, WTP = Water Treatment Plant
Traditionally, WWTPs equipped with anaerobic digestion and utilisation of produced biogas reach 50 – 70 percent self-sufficiency in electricity, on average. In Viikinmäki, >90 percent self-sufficiency was reached mainly by i) procurement of new CHP units with higher capacity and better efficiency; ii) deammonification of sludge treatment rejects, which reduced the need for aeration energy in wastewater treatment; and iii) implementation of ORC units, which utilize heat from CHP exhaust for electricity generation. Further actions implemented recently or in the pipeline of the next one to two years include increase of diffuser density in aeration; increased TS content in digester feed; improved heat recovery from digested sludge; and increase of biogas storage capacity. We expect they will elevate our electrical self-sufficiency permanently above 100 percent by 2025.
Due to the variability of wastewater flow, load, and temperature, the rates of energy production and consumption at the WWTP are constantly changing. In order to combine optimal production, use, and purchase of energy, certain features have been implemented. About 0,7 MW of electricity and 0 –0,4 MW of produced heat is constantly directed from Viikinmäki WWTP to the nearby Vanhakaupunki drinking water production plant. In case electricity production exceeds the sum of the WWTP’s consumption and the transfer to Vanhakaupunki, up to one MW of the surplus can be sold to the market.
In addition, spot prices of electricity are continuously monitored by the plant’s SCADA system. When the price is low, typically during night-time and holidays, own production is tuned down, biogas storage is filled, and cheap electricity is bought in from the market. Conversely, in the hours of high price, own production is maximized by running the CHPs in full capacity, creating a surplus of electricity, which can be sold to the market. The system is automatic, operating without any operator intervention. Itis a good example of fairly simple innovation initiated by changes in the environment, designed, and implemented by HSY’s own personnel during the energy crisis of late 2022. During its first three months of application, the system created savings of more than 100,000 EUR.
Future Prospects
The new Blominmäki WWTP, commissioned in 2023, has several unique features for optimising the combination of heat and energy generation and utilization. We aim to present the results of their first year of operation, as well as those of the most recent improvements in Viikinmäki, in another article in 2024.
