Cost-effective Operation of Wastewater Treatment Tanks

According to a study by the German Environment Agency , wastewater treatment plants are responsible for around 1% of Germany's total electricity consumption. What initially sounds like a very small percentage, however, offers a highly attractive savings potential from the point of view of individual wastewater treatment plant operators. One typical example of the savings potential in wastewater treatment plants is tank aeration. For example, state-of-the-art blower technologies and demand-driven control technology can energetically optimise the energy-intensive process of tank aeration and significantly increase a treatment tank's economic efficiency with comparatively little effort.

This starting point is particularly interesting for cities and municipalities. A glance at the energy requirements of municipal wastewater treatment tank operators shows that wastewater treatment plants in towns and municipalities account for a considerable share (around 20%) of total electricity consumption. Water treatment thus plays a crucial role when compared to schools, hospitals, water supply systems or other municipal and urban energy consumers.

However, the energy saving potential of wastewater treatment plants is often underestimated. Many wastewater treatment plants are still operated today with unregulated and inefficient aeration systems, which are responsible for a large part of the total electricity consumption. From today's point of view, however, the unregulated and non-demand-driven aeration in wastewater treatment plants is technically outdated. Highly efficient systems such as turbo blowers, rotary blowers and rotary compressors have been available on the market for several years. These not only offer higher efficiency in their own right and can be operated in a wide partial load range, but can even be operated together for optimum overall efficiency. This makes it possible to optimally respond to load changes in a tank's demand profile and to sustainably reduce energy costs.

In our guide, we will not only discuss the importance of aeration technology for the economic efficiency of wastewater treatment plants, but also point out the potential of demand-driven blower technologies. In addition, we will show how operators of wastewater treatment tanks can optimise their aeration tanks, from the initial analysis through to interlocking control.


The importance of aeration technology for the economic efficiency of wastewater treatment tanks

Every year, wastewater treatment plants consume a gigantic quantity of electricity amounting to around 4,400 gigawatt hours (GWh) of electrical energy . This is roughly the same as the annual output of a modern coal-fired power plant and equates to an annual CO2 production of around 3 million tons. Hence increasing the energy efficiency of wastewater treatment plants not only benefits operators – often the town/city or municipality – but also the environment.

When it comes to making the processes in wastewater treatment plants more energy-efficient, aeration is the main focus of consideration. In many cases, the energy requirement of the blowers installed in the aeration tank amounts to 60 to 80 percent of the total requirement. But what significance does aeration technology actually have for wastewater treatment plants?

Aeration tanks constitute the main cleaning stage of wastewater treatment plants. They use aeration systems to introduce oxygen from the air into the tank to promote the growth of aerobic microorganisms. These microorganisms break down organic pollutants and build up pollutant-absorbing activated sludge – this activated sludge is then separated from the treated wastewater in secondary treatment tanks and returned to the activated sludge tank or used as sewage sludge. In addition, the introduced oxygen is responsible for oxidising the ammonium nitrogen in the wastewater to nitrate and thus preparing it for removal by microorganisms.  

The energy consumption of wastewater treatment plants is usually expressed in kilowatt hours per year per PE, where PE stands for population equivalent. Typically, this value decreases as the size of the wastewater treatment plant increases, as economies of scale can be exploited. The German Environment Agency provides orders of magnitude for the electricity consumption of wastewater treatment plants that can be used for orientation purposes:

  POPULATION EQUIVALENT SPECIFIC POWER CONSUMPTION
Size class 1 < 1,000 75 kWh/(EW x a)
Size class 2 1,000 – 5,000 55 kWh/(EW x a)
Size class 3 5,000 – 10,000 44 kWh/(EW x a)
Size class 4 10,000 – 100,000 35 kWh/(EW x a)
Size class 5 > 100,000 32 kWh/(EW x a)

It is interesting to note that, despite their lower specific energy requirements, the wastewater treatment plants of size classes 4 and 5 are responsible for around 87 percent of total electricity consumption. Wastewater treatment plants of this size represent only about 22 percent of the approximately 10,000 wastewater treatment plants, but are responsible for over 90 percent of the population equivalents.

In a typical wastewater treatment plant, biological treatment and secondary treatment form the largest part of the energy requirement. However, other process steps also require electrical energy:

  • Biological cleaning and secondary treatment 67%
  • Sludge treatment 11%
  • Flocculation filtration 8%
  • Infrastructure and other consumers 6%
  • Wastewater pumping systems 5%
  • Mechanical cleaning stage 3%

For this reason, the energy optimisation of waste water treatment focuses primarily on the biological treatment stage. A comprehensive analysis of 85 wastewater treatment plants in North Rhine-Westphalia showed that optimisation measures in the aeration tank were particularly effective: they created around twice the savings as measures taken in other areas.

There are manifold possibilities for technically and energetically optimising the biological cleaning stage. They range from the replacement of the blower system to the targeted improvement of operational management and the avoidance of pressure losses, to investments in highly efficient pump technology with low susceptibility to clogging.


Base load, peak load and low load – supply when needed with modern ventilation technology

The aeration tanks of municipal and industrial wastewater treatment plants are characterised to a large extent by a fluctuating load profile. Fluctuating quantities of waste water during the course of the day, changing precipitation levels, seasonal effects and varying degrees of pollution in the waste water result in a fluctuating demand profile, which can sometimes present sudden load changes. In addition, seasonal temperature differences must be taken into account, which also have an influence on the required amount of atmospheric oxygen in the activated sludge tank.

Modern concepts for the aeration of activated sludge tanks therefore aim to respond to a wastewater treatment plant's fluctuating demand profile with the most efficient aeration technology for each individual case. The advantages of turbo blowers, positive displacement blowers and rotary lobe compressors can thus be used in a targeted way to operate the ventilation system at efficient operating points, both at base load demand and at peak and low load demand.

While the turbo machine, for example, has excellent efficiency in terms of design, the positive displacement machine can be used in a wide partial load range with good efficiencies. If the advantages of both technologies are used, an aeration concept is created that can be ideally tailored to the individual requirements of the wastewater treatment plant and represents the best possible solution for the customer.

For this reason, AERZEN has developed a composite concept that enables bespoke and highly efficient oxygen supply at any time. The three-product range, also known by catchy name of "Performance³", enables operators of wastewater treatment plants to combine the best possible efficiency with maximum operational safety, reliability and economy.

The three series are specially designed for demand-driven supply of basic, peak and low loads. The AT and TB turbo blower series are characterised by outstanding efficiency in terms of design and are therefore primarily used for the base load range. To generate peak and low loads, AERZEN enlists the adjustable positive displacement blowers from the Delta Blower series and the rotary lobe compressors of the Delta Hybrid type.


The technical data of the Performance³ portfolio at a glance:

Technology Series Volume flow Pressure range Control range
Turbo blower AT and TB 300 – 16,200 m³/h 400 – 1,000 mbar 40 – 100 %
Positive displacement blower Delta Blower 30 – 15,000 m³/h -500 – 1,000 mbar 25 – 100 %
Rotary lobe compressor Delta Hybrid 110 – 9,000 m³/h -900 – 1,500 mbar 25 – 100 %
To enable targeted use of an individual series' characteristic advantages, it is worthwhile to optimise the control technology of the overall system. The main objective here is to find the optimum mode of operation for the activated sludge tank – by carrying out an in-depth analysis and evaluation of the oxygen demand – and to implement this with intelligent control technology.

Becoming an efficient wastewater treatment tank, step by step

To enable an evaluation of the energetic improvement potential of the aeration technology in wastewater treatment tanks on a sound basis, it is first necessary to carry out a detailed analysis of the existing aeration technology as well as the load profile. Extensive measurements and analyses make it possible to identify and quantify the potential of energetic modernisation measures. With the innovative AERaudit service from AERZEN, it is possible to optimise the usage and efficiency of blower stations in a targeted manner.

Following this analysis phase, the ventilation concept is implemented with the aid of a higher-level control system. Systems such as the innovative AERsmart machine control system from AERZEN are able to distribute air volumes across the technologies and their individual efficiencies in an ideal way, thus sustainably increasing the efficiency of the plant and bringing it close to its theoretical optimum. By simply optimising the control system, a considerable savings potential of up to 15% can be achieved. In addition to the proprietary Performance³ series, third-party products can be also controlled.

The unique feature of the AERsmart interlocking control system is its ability to distribute the required oxygen demand to the machinery at any time in such a way that each load point can be driven at the most efficient operating point. The controller uses the respective characteristic maps and efficiencies of the blowers and selects the combination of blower technologies that is ideally suited for the current operating point.


Resource-conserving wastewater treatment using Holzkirchen as an example

The example of the Holzkirchen wastewater treatment plant impressively demonstrates how thorough wastewater treatment can also be carried out in an environmentally friendly and resource-conserving manner. The wastewater treatment plant near Munich is responsible for around 50,000 PE and consumes around 500,000 kWh of electricity every year. By integrating the AERsmart integrated control system, the Holzkirchen wastewater treatment plant has succeeded in making ideal use of the advantages of various blower technologies. The result: An energy saving of around 10% and a reduction in annual electricity requirements of around 50,000 kWh.


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