Optimum Planning in Plant Construction Ensures Efficient Wastewater Treatment

Biological treatment in the aeration tank is not only the key stage in prevalent wastewater treatment processes, it is also the most energy-intensive. If operators want to improve the energetic balance of their wastewater treatment plant, optimising the aeration in the aeration tanks gives them a large amount of design leeway. The correct dimensioning of the aerators, the selection of the appropriate technologies and the intelligent data-guided control of the operation in the aeration tank are therefore the focus when it comes to plant construction. Energy consumption and emissions can be significantly reduced through integrated planning in new project planning and refurbishment.

Factors for efficient wastewater treatment

For holistic work of the wastewater treatment to be successful, it is crucial that all relevant target dimensions are sufficiently considered:

  • Cleaning performance
  • Operational stability
  • Costs

As a measure for the quality of wastewater treatment, cleaning performance represents the highest priority with a wastewater treatment plant. After all, it is necessary to comply with applicable legal limits which, depending on the use of the treated water, may even exceed the minimum requirements of the German Wastewater Ordinance. The plant's stability should be viewed in a similar context. A wastewater treatment plant must achieve the required performance data even under shock loads. However, technology that has exceeded its service life and operating modes with high material wear can endanger this.

Energy costs are almost the only cost factor that can be significantly influenced in the day-to-day operation of a wastewater treatment plant. Other costs, such as for operating personnel or maintenance, should be regarded as fixed as far as possible. When building a new plant, crucial decisions for the later energy demand are made in the early phases. However, with around 10,000 municipal wastewater treatment plants nationwide, new construction is clearly an exception.

Therefore, replacing plant components is much more frequently the case. In practice, however, this usually only happens when it becomes necessary to replace defective technology – in very few cases is this "only" carried out to optimise cost structures. Anyone who wants to effectively reduce the running costs of wastewater operation should nevertheless start with the energy requirement, where significant improvements can be achieved even without replacing larger plant components.

The design of the plant configuration and the choice of operating parameters also have an impact on other important factors such as the nature and quantity of the wastewater sludge and the possibilities for nutrient recovery.

Wastewater treatment: Energetic aspects

Wastewater treatment plants account for around 20 percent of the energy consumption of municipal facilities. This shows what a beneficial impact energy savings can have on a municipality's finances. However, the fact that wastewater treatment is energy-intensive and therefore represents a large savings potential is also true of commercial plants. In addition to the financial perspective, there is the ecological perspective: In a study conducted by the Federal Environment Agency, operators can achieve average savings of 30 percent without investing in new machinery, thus avoiding emissions of around 900,000 tonnes of CO2 per year.

The aeration of wastewater in the aeration tank, where the biological treatment is carried out, accounts for approx. 50 percent of a wastewater treatment plant's energy consumption. In addition, plants with wastewater sludge treatment have constantly running agitators in continuous operation. The other process stages each account for a relatively small proportion of total energy consumption and have correspondingly lower priority in plant construction optimisation projects.

However, wastewater treatment plants are not just consumers of energy; in many cases, they also produce it. The wastewater treatment plants of size classes 4 and 5, which in Germany account for the majority of the population (PE), pump wastewater sludge (a by-product of wastewater treatment) into what are known as digestion towers. With the addition of heat energy, the decomposition of the sludge's organic components produces thermally usable digester gas.

Block-type thermal power stations convert the gas into electricity and heat. Reducing energy requirements is therefore not the only starting point for improving efficiency. In addition, there is also great potential in increasing one's own electricity and heat yield and thus reducing externally procured energy for the operation of blowers, pumps and agitators as well as for heating the digester tank. AERZEN also offers solutions and concepts for this comprehensive approach to plant construction and integrates energy generation into the planning.

Aeration as a focal point in plant construction

The aeration of an activated aeration tank required for biological treatment accounts for around 50 percent of the total energy consumption of large wastewater treatment plants, and the proportion tends to be even higher for smaller plants. Efforts to reduce running costs should therefore begin here. Whether it be new planning or replacement investment, expert adjustment of the system configuration to the wastewater treatment plant's individual requirements can achieve significant improvements. Blower technology, number and size of the individual blowers and intelligent control of them offer considerable scope for design. AERZEN has been working closely with operators of wastewater treatment plants for decades and has incorporated these requirements into the development of products and solutions.

To support customers with the overall optimisation of their aeration, AERZEN not only offers highly developed blower technology as part of the Performance3 concept, but also the corresponding control software and the expertise for coordinating the system. AERZEN supports customers as early as the planning phase and works out the corresponding drafts together with them. Experience from a wide range of projects and specially-developed planning aids help to find a fast, prudent and future-proof solution.

The choice of aeration system determines the energy requirement of the biological treatment stage over a considerable period of time. This is because the blower stations can often only be replaced as part of a replacement investment. In both planning cases, i.e. new construction and renovation, it is therefore crucial to select the optimum technology and the suitable model. For technical reasons, different blower technologies have different operating characteristics and the size of the individual units also plays an important role.

AERZEN offers its customers the Delta Blower positive displacement blower, the Delta Hybrid rotary lobe compressor and the turbo blower Aerzen Turbo. While turbo blowers are particularly energy-efficient at the design point, their load range is less broad than with other technologies. In contrast, a rotary lobe machine has a larger control range. The system can therefore handle changing loads very efficiently because the efficiency remains almost the same at all operating points.

This means that different blower technologies are available with individual strengths; the task of the system planner is to configure them into an aeration system in a way that combines their strengths and optimally serves all load profiles. The option of operating differently-sized assemblies together and arranging them in different ways creates further planning freedom. The AERZEN application specialists support the analysis of consumption and load profiles in order to provide a data basis for further planning.

The basis for the optimal use of this gain in flexibility, however, is a sufficient data foundation. The lack of information about the operating conditions of the relevant plant components and the load characteristics of the wastewater treatment plant make it difficult to realise the potential of modern plant technology.

While the operation of wastewater treatment plants has generally been strongly influenced by the experience of its staff, the use of the existing efficiency potentials requires a rethink. This particularly applies to the aeration stage, where changing loads and blower stations, which are made up of assemblies with different characteristics, make intelligent data-based controls necessary. Monitoring the power consumption of the most energy-intensive plant components and measuring pressure losses is a prerequisite for energy optimisation and early detection of faults. That said, data collection is not yet standard, especially in smaller wastewater treatment plants. However, falling prices for the relevant measurement technology make retrofitting easier.

When reconfiguring the blower station and defining the individual control strategy, data acquisition is an integral part of the planning process. During an AERaudit, the experts from AERZEN record the development of volume flow, system pressure, temperatures and power consumption over time. This data allows comprehensive analyses of the aeration tank's load profile. On this basis, the plant construction specialists develop a plant concept that is optimally adapted to the individual characteristics of the wastewater plant. The aim in this regard is to expand the characteristics of the different blower technologies and assembly sizes in an advantageous manner.

The plant control system determines which of the compound's blowers should work at which operating point at a given total load of the wastewater treatment plant. The AERsmart integrated control system distributes the volume flow to the individual machines in a way that ensures their efficiency levels are as optimised as possible. This enables the control system to react at an early stage to load changes with high wastewater volumes and high levels of contamination. The global control system can achieve energy savings of up to 15 percent. By using the state-of-the-art blower technology, plant operators can reduce aeration energy by up to 50 percent. However, the development of a coherent overall concept by the plant manufacturer is always the decisive factor.

In plant construction, flexibility is in demand

The energetic optimisation of aeration enables considerable reduction of a wastewater treatment plant's energy costs and emissions. Implementation is often difficult in practice, and especially in municipal institutions. Although investments in blower technology quickly pay for themselves thanks to the high efficiency gains, getting the funds released presents an obstacle to implementation.

A blower station can often only be replaced by plant constructors as part of an already necessary replacement procurement. This makes Performance3 from AERZEN particularly flexible, meaning that all wastewater treatment plants can improve their energy efficiency. In addition to the latest blower technologies from the company's own portfolio, existing units from third-party manufacturers can also be integrated into the smart global control system. Of course, the control unit can also manage a single machine. Even system operators who are unable to completely replace the aeration unit in the medium term can thus achieve double-digit percentage improvements in energy efficiency.

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