Heat Recovery from Wastewater Treatment - The Best Practices

For heat recovery to be cost effective, it is essential for processes to be well coordinated and the cooling water discharge temperature needed for the process must be reached. Examples for use of cooling water from waste heat or heat exchangers: sludge drying (40 to 50 °C), hot water forward flow (40 to 60 °C), pre-heating of hot water and supply air (30 to 50 °C), and building heat forward flow (60 to 85 °C).

Usable Temperature Level

ΔT between waste heat source and heat sink (consumer) should be at least 5 to 10 K – the higher the better.

Heat Quantity and Heat Output

The recovered energy must be perfect for the process that is in use, to prevent the need for extra heat generators during peak demand.

Continuity of Use

Heat recovery can also be worthwhile in systems with low waste heat if they attain a high utilization of capacities.

Physical Proximity

It is essential for the waste heat source and heat sink to be as close to each other as possible, in order to minimize transport costs and losses.

Synchronization of Operating Times

Exact synchronization of the operating times of heat sink and the heat source ensures more efficient use of the waste heat. If demand and supply are not completely synchronous, a heat accumulator could be a good solution.

Operating Hours and Service Life

The economic benefits are higher when the system is in operation for longer and the utilization of the available capacities is higher.

Investment Costs

The investment costs are directly affected by factors such as heat transport and storage.

Reliability of Supply

If a sensitive process uses just waste heat, it may be essential to provide a backup heat generation system in case there is a failure of the waste heat source.

Methods of Heat Recovery

Air-cooled blowers, compressors or turbos with an acoustic hood are perfect for exhaust-air heating of rooms. For this purpose, the exhaust air louvre of the AERZEN unit is provided with an exhaust duct. This permits use of the cooling air for the silencer, the compressor stage and the pipe system under the acoustic hood, while the exhaust air from the oil cooler can be used for room heating. The waste heat, at a temperature of 30 to 60 °C, is concentrated through the exhaust duct and can then be guided through air ducts into the rooms that are to be heated.

Temperature-controlled flaps are employed for regulating the room temperature. The discharge-side gas flow itself proves to be more promising for heat recovery. It comprises of up to 85% of the electrical energy in the form of heat. It is possible to recover this energy by means of a compact heat exchanger, which is installed in the piping downstream from the compressor (on the discharge side).

Type reference & life cycles (operating hours). Image Credit: Aerzen

Wastewater Treatment

In wastewater treatment plants, blowers, Delta Hybrid compressors and turbos that reach maximum air discharge temperatures of 140 °C are used. A condensate trap is not needed. Wastewater treatment plants are considered to be the facilities in municipalities and cities that use the most electricity. Thus, due to widespread uncertainty concerning the future global energy matrix and the resulting energy market price, there is an urgent requirement to increase energy efficiency.

By using a heat exchanger with a low differential pressure (<30 mbar) the blower station consumes just slightly more energy than normal (<2 % of the operating energy input). On the other hand, the energy saved is many times higher than the extra input power needed. Heat recovery systems for wastewater treatment plants, example, for optimal support of the sludge-drying process, are normally designed to produce the return flow of cooling water from the heat exchanger at >70 °C at the maximum heat transfer rate.

Case 1: hot water > 70 °C
Case 2: increased cooling water flow
Case 3: increased cooling water flow and transferred heat quantity optimized

Blower station. Image Credit: Aerzen

An Annual Load Duration Curve and Partial Load

An annual load duration curve can be used for estimating the percentage of required heat that can be achieved using a system designed for partial load operation. This example of a heating system points out that the peak capacity is required only for a few hours during the year. It is possible to cover 50 to 70 % of the heat requirement in a system designed for 15 to 25 % of the power requirement. Other applications must be analyzed individually.

Available Heat Quantities

Optimal waste-heat recovery needs a determination of the available heat quantity. This relies on the mass flow/volume flow, usable temperature difference, the specific thermal capacity of the heat transfer medium, and the time of availability.

Operating hours of the consumer per year. Image Credit: Aerzen

Features of the Aftercooler

Function: compressed medium flows via the cooler’s pipes, cooling water flows through the pipes in a counterflow. The louvre design of the tubes guarantees effective heat transfer and minimal pressure loss.

Variants: Permanently installed or removable pipe bundles, ribbed or smooth pipes, stainless steel for high gas temperatures, nickel/copper for seawater.

Accessories:

• Automatic condensate drain
• Flange and counter flange kits
• Cyclone separator

Aerzen's design tool lets you quickly find the best heat exchanger for your system. Image Credit: Aerzen

Explanations of the Type Designation

Image Credit: Aerzen

ARN removable pipe bundle
AFN fixed pipe bundle

Image Credit: Aerzen

A = cooler length (without flanges) in mm
B = flange width in mm
C = distance between the water connections in mm
The exact dimensions will be determined during the configuration phase.

This information has been sourced, reviewed and adapted from materials provided by Aerzen Machines Ltd.

For more information on this source, please visit Aerzen Machines Ltd.