Wastewater heat recovery is worth it

Wastewater heat recovery systems are probably the least known energy-saving system in Germany. Even the word itself is complicated, the functionality a little less. The only question is whether this system can actually be worthwhile in-house, and a use makes sense. Here we find answers to these questions, and at the same time explain how it works.

Functional principle of wastewater heat recovery

Per capita, we consume around 120 liters of water per day in Germany. Not a small part of it is warm water:

  • Water is heated by the washing machine
  • Water is heated while cooking
  • Water is heated for bathing and showering
  • the dishwasher works with hot water

The water consumption of the washing machine and the water consumption of the dishwasher together are quite significant. Added to this is the water consumption during showering and personal hygiene, which together accounts for almost 70 to 80 percent of our drinking water consumption, ie almost 100 liters of warm water per day and person.

Recovery of lost heat energy

The water runs into the canal after use. However, energy is used to heat the water - from the appliances and from a water heating system. This energy needs to be at least partially recovered.

The hot water has a high average temperature after leaving the house. After all, stuck at this time still around 25° C, unused heat in the water. Even in the channel you could get 15° C of heat from the sewage.

With this heat you could heat, or at least support the heating system effectively. To what extent, we will discuss in detail.

Systems for use

A so-called AWNA (sewage heat utilization system) is not a new invention. These devices have been on the market since 1982. Nevertheless, they are hardly known.

Technical problems with the use of classical systems

For commercial use in the sewer, there is the problem that conventional heat exchanger plates would become clogged by the solids and feces contained in the effluent. For use behind individual buildings, the technical problem is the fluctuation of the wastewater attack.

Unless a particular continuous volume of waste water is available, the recovery of heat from the wastewater is not continuous - making it technically difficult to exploit.

Such systems are only suitable for larger buildings, for example, from 25 residential units. The waste water is collected there centrally before it is directed into the sewer. For individual households, these systems are therefore of no interest.

The collected wastewater serves as a heat storage, which is then slowly removed from the outside via heat exchanger plates and returned to the building. However, this requires extensive structural measures and planning. In addition, filtration systems must be installed, which must be backwashed again and again, and the entire building technology to be matched to this system, as well as the sewer connection.

Downspout heat exchanger

The disadvantages of large plants are elegantly bypassed at the downpipe heat exchanger. Water flowing through a vertical pipe forms a thin film with a large surface at the pipe surface. There, the heat of the water can be easily removed because of the large surface area. Additional energy is not required, and there is no need for maintenance. With the heat removed, the cold water can be warmed up.

Economy of the downpipe heat exchanger

Up to 35 percent of the energy can be saved for heating up the cold water. In individual cases, this always has to be compared with the acquisition costs according to the respective situation.

Even large systems often pay off very quickly. Here, up to two-thirds of the wastewater heat can be utilized, if the appropriate structural measures are taken. In the case of the merger of several households (for example, in a settlement), this can be an interesting opportunity to significantly improve the energy balance of all connected houses. Here there are often payback periods of 5 years and less given for the plant.

Video Board: Drain Water Heat Recovery Systems