Energy is mainly produced from fossil (oil, gas and coal) and fissile (nuclear) energies which have limited reserves. Some 10 billion toe (tonnes of oil equivalent) are produced each year. Due to the econo-mic development of countries, a 50 to 300% increase in consump-tion is expected by 2050.Water treatment will contribute to this increase. The scarcity of fresh-water resources and deterioration of water quality, combined with increasingly strict regulations, will not only require more sophistica-ted water and sludge treatment but also more sophisticated pro-cesses that tend to consume more energy, the price of which is constantly rising.At the same time, the use of fossil fuels contributes to global war-ming through the emission of greenhouse gases.

ENERGY EFFICIENCYWhat does it mean?

Implementing energy efficiency entails reducing and optimising the energy consumption of a process while maintaining its efficiency. This approach has financial and environmental benefits. The possi-bility of producing green energy – and using it to power facilities directly or supplying energy to grids – reduces the greenhouse gas emissions and carbon footprint of plants, cuts their energy bill and their dependence on energy prices.

The European Union has adopted Directive 2012/27/EU which sets the objective of «increasing energy efficiency in the European Union to achieve the objective of saving 20% of the Union’s primary energy consumption by 2020 compared to projections”.

WATER TREATMENT AND ENERGY CONSUMPTION

Conventional production of drinking water

The treatment for the production of drinking water is no longer ener-gy-intensive compared to the treatment of wastewater. However, with the deterioration of the quality of resources and the increasin-gly advanced search to remove certain compounds, through ultra-filtration for example, the energy used to treat water is tending to increase.Upstream and downstream pumping is the largest energy consumer and needs to be systematically optimised according to the local set-ting up (especially in mountainous areas for example).Concerning the treatment itself, energy requirements range from 30 Wh/m3 (classic treatment) to 120 Wh/m3 (activated carbon and ozone), and 200 Wh/m3 for membrane ultrafiltration.For membrane technologies, a key point for energy optimisation lies in the design of pre-treatments to reduce waste discharge from membrane treatment (2-3% in the case of ultrafiltration, 15% for nanofiltration).

Seawater desalination

There are two types of desalination process used on an industrial scale to produce fresh water from seawater: thermal and membrane processes. The energy requirements vary according to the process used.Distillation processes require a substantial amount of thermal ener-gy. They need to be coupled with other heat-producing applications which should be located nearby.The energy needed for reverse osmosis can be broken down into three functions: carrying out pre-treatment, overcoming osmotic pressure across the membrane, and overcoming the membrane’s resistance to the flow of water.Essentially derived from electricity, the energy consumed by the reverse osmosis process allows it to be coupled with a more exten-sive energy source (such as wind or photovoltaic). Because reverse osmosis desalination plants are powered by the electrical grid, they can be sited close to raw water sources and water consumers.

Energy consumption specific to reverse osmosis is relatively constant depending on the treatment line. It is around 4,000 Wh/ m3 of water produced. With a particularly poor quality raw water (or-ganic matter or algae) and a lower temperature, consumption can reach 7,000 Wh/m3 given the energy required to carry out pre-treat-ment requiring a series of stages. The treatment of boron, if any, will increase it by a further 700 Wh/m3.Energy expenditure represents over 50% of operating costs and 20% of the price of the treated water. Over 60% of the energy is used during the first pass of reverse osmosis.In 40 years, energy requirements for seawater desalination have decreased considerably thanks to the implementation of energy recovery systems, improved pump yields and the development of new membranes.

Treatment of wastewater

The energy consumption of a wastewater treatment plant depends on the required quality of the discharge.Two discharge criteria have led to an increase in specific consumption:- moving from the treatment of carbon alone (COD and BOD5) to include nitrogen (TN);

Degrémont Water Treatment Handbook Factsheets

ENERGY EFFICIENCYWATER TREATMENT OFFERS POTENTIAL SAVINGS

Drinking water

Re-UseUrban

wastewater

Industrial water production and treatment

DesalinationBiosolids

Degrémont has a tradition of sharing its employees’ passion for water treatment with the public.To supplement the Water Treatment

Handbooks, Degrémont has issued the «Handbook Factsheets» to promote a better

understanding of the different techniques available and discovery of the new products and major technological changes.

Electricity consumption for a first pass of reverse osmosis

- moving to high bacteriological quality and the removal of suspended solids.Processes are continuously optimised to reduce the energy impact based on improvements in treatment performance.The breakdown of electricity consumption (see chart opposite)

shows the substantial propor-tion of biological treatment in the overall consumption of energy. Energy optimisation for aeration and the rough-ness in tanks is critical.

Reuse of treated wastewater

For the majority of applications that reuse treated wastewater such as irrigation or the watering of golf courses and public gardens, ener-gy consumption is low. However, it increases according to the quality required by the final use of the water which determines the number of treatment stages and their sophistication. For example, the energy required to implement reverse osmosis is higher than that required for conventional processes. It can be about 75% higher than for treat-ment by membrane bioreactor.

Treatment of sludge from wastewater treat-ment plants

• Energy potential of sludgeThe energy poten-tial of sludge repre-sents more than twice the electricity consumption of a wastewater treat-ment plant. Sludge is much more than waste; it repre-sents a re-usable resource. Digestion and green incineration allow energy savings or energy production.

Energy consumption for sludge treatment depends on the type of technology used. In general, for sludge from extended aeration, al-most all treatment types are more energy-intensive than for mixed sludge:- digestion, because the removal efficiency of volatile matter (VM) is much lower;- drying and incineration since there is a higher quantity of water to be evaporated as the dryness obtained through dehydration is less for sludge from extended aeration.

• Energy potential recoverable through digestionWith mixed sludge, digestion transforms 50% of all VM into biogas.With extended aeration sludge, the yield is 30%. The production ratio is 1 Nm3 of biogas per kg of digested VM, the net calorific value (NCV) of the Nm3 thus being 6.3 kWh/Nm3.The “boost” of the digestion prepares the organic matter for bet-ter productivity of the methanisation phase and can significantly increase the production of biogas.

• Calorific energy potential recoverable through incinerationThrough incineration, 100% of the calorific potential of the VM of sludge is used to evaporate the water contained in the humid matter. If this energy is not sufficient, an external source of calorific energy is necessary. As the evaporation of 1 kg of water requires 1 kWh of thermal energy, the dryness of the sludge (dehydrated or dried) is essential in incineration. The flue gas from incineration also offers heat recovery potential.

ENERGY EFFICIENCY TO BENEFIT WATER TREATMENT

Thanks to its capacity for innovation and expertise in engineering, Degrémont has developed a comprehensive range of solutions to improve the energy efficiency of all the processes involved in wa-ter treatment. The company’s energy optimisation policy seeks to strengthen best practice in its activities (from the development of its processes in R&D through to the design of its plants), in the ma-nagement of operations under its responsibility, and the equipment it supplies. Degrémont responds to the energy and environmental concerns of its local authority and industrial clients in compliance with the regulations that impose rigorous health and safety require-ments on the production of drinking water and advanced treatment for purifying wastewater and treating sludge. Whenever possible, Degrémont incorporates renewable energy in its projects.

Pumps

Pumps are integral to all water treatment sectors (production of drin-king water, treatment of wastewater, desalination, etc.). The energy consumption of pumps accounts for the major part of their overall cost (capital expenditure + operation + maintenance + replacement) throughout their period of use.Through close collaboration with leading pump suppliers, Degré-mont has developed a decision-making tool which selects pumps whose optimum yield point corresponds to the needs of the project and it compares them according to purchase price, energy expense and maintenance cost based on their overall life cycle. The tool also compares the best solutions according to pipe diameter in order to facilitate the selection of THE best solution.Prerotation and/or vortex phenomena are factored into the design of pumping stations to avoid energy waste; likewise for hydraulics, of-ten the second biggest energy consumption item at treatment plants and consequently the second source of energy savings.Optimisation are made during the project design stage. Degrémont optimises each plant with the most appropriate piezo-metric line taking the specifics of each site into account, to reduce the energy needed for pumping and maximising the use of differences in alti-tude offered by the site.

Treatment of wastewater

• Biological aerationBiological treatment requires large amounts of air to feed the pu-rifying bacteria. Biological treatment accounts for 40 to 70% of a plant’s energy consumption.To ensure reliable and effective production and circulation of air, eve-rything has to be taken into account, from the design of the aeration tanks to the choice of air production machines, the type of sparger and the system for regulating the biological aeration. Note that only improving one of the aspects of the aeration system would not be sufficient to obtain a system that is efficient overall.

http://www.degremont.com/efficacite-energetique/

- Design of the tanks: good design for tanks leads to efficient mixing within them and less need for air.

- Regulating aeration: an efficient system for regulating aeration will respond to the exact needs of the biological treatment and the water treatment will be of high quality without excess energy consumption. Many plants only monitor dissolved oxygen and/or the redox potential for controlling aeration. Degrémont has de-veloped an original control system cal-led Greenbass™ which simultaneous-ly utilises N-NH4

+ and N-NO3

- concentra-tions to control the process of nitrifica-tion / denitrification with greater precision.Greenbass™ adjusts the aeration to meet the exact air requirement of the bacteria in the aeration tanks. Consequently, there is no excess aeration and the water treated is always of very high quality. This sys-tem is currently being used at a dozen plants and has reduced their energy consumption by up to 15%.

- Choice of equipment: the choice of air spargers, air production machines, valves, etc. can contribute to cutting energy consumption, with precautions taken to ensure that the quality of the treated water is not compromised. Degrémont has implemented new air produc-tion control strategies which decrease energy consumption at the same time as improving the quality of the water treatment. If well chosen, air production machines can enable savings of up to 30% of the energy consumed by aeration. To provide clients with the less as possible energy consumption wastewater treatment plant, Degré-mont has developed a selection tool for air production machines.This tool, based on a calculation of the life cycle cost (energy re-presents between 80 and 95% of the total life cycle cost over 10 years), quickly and efficiently identifies the best technology to mini-mise energy consumption linked to the production of air in biological tanks. For the calculation of energy consumption, Degrémont prefers to use the “Wh/Nm3/100 mbar” ratio. This ratio enables the efficiency of different machines to be compared individually, at the same time as providing an overview of the average efficiency of a technolo-gy. The tool selects the technologies that meet specific needs and automatically provides a clear and readily appreciable comparison between the machines. It produces helpful charts.

Energy in sludge

Degrémont has developed technologies which enable the energy “contained” in the sludge to be recovered and reused in situ, notably via the biogas produced during digestion of the sludge or the heat released during its incineration. The aim is to achieve energy auto-nomy.

• Digestion: the principal advantage of digestion in terms of ener-gy efficiency is the pro-duction of green energy through the biogas gene-rated.Digelis™ Turbo is a De-grémont technology for boosting digestion and improving the production of biogas thanks to a pri-mary process for thermal hydrolysis which breaks down the cellular structure of bacteria and dissolves the exopolymers into a digestible form.

The fruit of a partnership with CAMBI, Digelis™ Turbo offers excellent yields and practically doubles the production of biogas compared to traditional digestion.

Ways of recovering energy from biogas- production of heat for heating the digesters and premises, and the supply of thermal energy for the dryers;- cogeneration: simultaneous production of electricity used on site or reinjected into the grid, and heat re-used on site for heating the premises and digesters, and supplying the dryers;- injection into the natural gas network after pre-treatment (proces-sing to biomethane);- production of biofuels: the biogas processed into biomethane enables the production of biofuels.

• Low consumption drying: drying sludge can account for up to 50% of a plant’s energy bill. 75% of the total cost in the life of a dryer is due to expenditure on energy.- Degrémont has developed EvaporisTM LE, a high-temperature low-consumption dryer which has the lowest energy input on the market. The process is based on the combination of direct and indirect drying , recovering heat between the two stages. Compared to traditional drying, this technology reduces energy consumption by 30%.

- With EvaporisTM LT, in partnership with STC, another Suez Envi-ronnement subsidiary, Degrémont has developed a low-temperature drying process aimed at reducing primary consumption through re-using the trapped thermal energy. The needs for fuel oil as a heat source are limited, the benefits are both environmental (limiting the need for fossil fuels and limiting greenhouse gas emissions) as well as economic.

• Advanced dehydration: DehydrisTM Twist implements a pro-cess that Degrémont has adapted from a BUCHER technology de-rived from the food industry. The process is totally automated and uses less energy than modern dehydration systems. The greater dry-ness obtained results in a smaller volume of dehydrated sludge, so less fuel is consumed in sludge drying and incineration.

Degrémont Water Treatment Handbook Factsheets

Production of Biogas in the digester for a 400,000 P.E. plant

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• Green incineration: a significant quantity of heat is available in the flue gas from sludge incineration. The possibility of producing energy through recovery of heat from flue gas makes it possible to envisage zero consumption of fossil energy.- A Degrémont process based on the preliminary drying of sludge using the heat in the flue gas from incineration before traditional incineration, ThermylisTM 2S has a very low environmental impact and does not require any fuel additive. A reference in this field is the Shenzhen wastewater treatment plant with 14,000 tonnes of CO2 per annum which are not discharged into the atmosphere.

- The second incineration technology recently developed by De-grémont is ThermylisTM 2S. The innovation of this process is the application of ORC (Organic Rankine Cycle) technology, patented by Degrémont. The available heat source is used to evaporate the organic fluid. Once vaporised to approximately 10-15 bar, this fluid is directed to a turbine to produce electricity, then channelled to the regenerator and condenser, before returning to the evaporator. The advantages include highly efficient electricity production and re-usable thermal power.

Air treatment

In a wastewater treatment plant, odour control is the third or fourth largest item on the energy consumption bill. It represents around 10% of the plant’s overall energy consumption, sometimes more depending on the facility.The main energy consumer is the fan. It should also be remembe-red that each component of the ventilation system, upstream and downstream of the fan, has an influence on the fan’s overall effi-ciency.Degrémont focuses on three key areas to maximise the efficiency of ventilation systems:- correct sizing of the air supply circuit;- limiting the quantity of foul air to be treated by prevention (limiting polluting emissions) or curative action;- reducing the quantity of energy needed to treat a given volume of air, by the selection of the fan and motor.

Desalination by reverse osmosis

Energy currently accounts for around 50% of the operating cost of a reverse osmosis desalination facility.• With help from its partners, Degrémont has contributed and conti-nues to contribute to controlling energy efficiency with regard to various factors, especially:- pumps;- improving pre-treatment techniques;-reverse osmosis (high-pressure pumping, membrane permeability, low-consumption membranes, conversion rate, etc);- energy-recovery systems and pressure exchangers from the concentrate;- wind energy.

• While continuing to optimise existing systems, Degrémont’s pro-posed future contributions to energy efficiency will focus on:- reducing water loss during pre-treatments;- increasing reverse osmosis first stage performances (via new mem-branes and new membrane engineering);- sources of alternative energy;- alternative processes to eliminate boron.

Energy monitoring tools

• Energy auditThe energy audit tool Energy Efficiency Rapid Diagnostic Tool (EERDT) enables the performance of wastewater treatment facili-ties to be evaluated; it is the first stage in establishing energy effi-ciency. The tool rapidly identifies strengths and weaknesses in terms of energy efficiency by structure or from an overall perspective. It enables consumption to be compared and analysed according to a benchmark derived from the operational experience of Degrémont and Suez Environnement.

• AQUACALC, developed by Degrémont, is a software management tool for water, sanitation and energy. Ope-rators can check the performance of their plant in real time. AQUACALC calculates practical high value-added indicators for monitoring, optimising and reporting on

the energy performance of facilities. AQUACALC enables the imple-mentation of ISO 50001 (Energy Measurement and Management Systems – Requirements and Recommendations for Implementa-tion) and the definition of Key Performance Indicators. The tool is already in use at over 110 plants worldwide.

Degrémont deploys its expertise and experience as a builder-operator at the service of energy efficiency and it designs plants adapted to individual situations according to local requirements and opportunities, additionally proposing:- energy-recovery technologies (e.g. heat pumps);- the implementation of hydraulic turbines;- recovering calorie from pressurised air or electrical rooms;- using renewable energy;- increased metering at facilities;- improving the maintenance of energy-intensive equipments;- very closely evaluating the processes needs ;- low-consumption buildings;- etc.

Contact : innovation.mailin@degremont.com

ERI pressure exchangers – Perth plant, Australia

Degrémont Water Treatment Handbook Factsheets