Choosing the right evaporator for wastewater and sludge

HRS has extensive experience in evaporating digestate, sludge and manure.

In order for evaporation to be both effective and energy (and therefore economically) efficient, a number of factors need to be considered, including the nature of the material to be evaporated; what is to be done with both the vapour or condensate and the remaining residue; the evaporation technique to be used; and the energy source/s available.

Whether you are looking to reduce the volume of sludge, extract valuable materials from the waste stream, or even implement a full Zero Liquid Discharge (ZLD) treatment solution, there is a wide range of equipment types and designs available from numerous manufacturers. This means that choosing the right solution, from the right supplier, can seem daunting, but posing the right questions at the beginning of the procurement and design process can prevent future misunderstandings and ensure that you arrive at the optimal product or system for your specific requirements.

Some of these key questions to ask any potential heat exchanger supplier include the following.

What evaporation method should I use?

Over the last couple of years, we have seen increased client interest in using mechanical vapour recompression (MVR) techniques for evaporation

Over the last couple of years, we have seen increased client interest in using mechanical vapour recompression (MVR) techniques for evaporation. Given the turbulent rise and fall of energy markets since 2020, this is understandable, as the electrical energy employed in MVR is normally considerably cheaper than the thermal energy needed for traditional evaporation. However, there are a number of limiting factors and key considerations when using MVR – particularly for very thick and viscous products like digestate and sludges – which can add to the capital cost (and complexity) of an MVR-based evaporation solution. It is therefore very important that all these elements are considered from the outset, so that an accurate investment decision based on both capital (Capex) and operational (Opex) costs can be made.

Digestates and sludges are challenging to evaporate because of their physical and chemical properties.

What are the differences between MVR and traditional evaporation?

Traditional evaporation techniques use a high temperature service fluid (such as pressurised steam) to raise the temperature of the product above its boiling point so that water (and other volatile compounds) is driven off, leaving a more concentrated solution. The principal source of energy for this process is therefore the fuel used to heat the water (steam) in the boiler, such as gas or oil.

In MVR, the steam which comes off the product in the evaporator is channelled into a compressor which increases the pressure (and therefore the temperature). This steam, which is now above the boiling point of the product, is then used as the service fluid for the evaporator. As the compressor uses an electric motor, the process is driven by electricity rather than thermal energy. Because the compressor reuses/recycles evaporated steam, a lot of latent heat is recovered. This makes MVR one of the cheapest methods of evaporating water in terms of operational costs.

There are several limiting factors when using MVR – particularly for very thick and viscous products like digestate and sludges

What are the limitations of MVR?

Because of the way MVR works, there are some inherent limitations in the process that traditional thermal evaporation does not suffer from. When evaporating at atmospheric pressure, depending on the type of compressor, the temperature rise provided by the compressor is typically between 8°C and 15°C. The relatively small temperature difference between the service fluid and the boiling point of the product (~100°C) means that heat transfer between the two is limited, and you need a large surface area to achieve it. To put it simply, you need a large heat exchanger.

In contrast, boilers can deliver maximum steam pressure of up to 8 or 10 bar, meaning an effective temperature of 160°C or 180°C. Even if general operation is below these levels, the fact is that steam from a boiler will be considerably hotter than from a compressor. The greater temperature difference means that less transfer surface area is required, and you can utilise a much smaller heat exchanger.

For high fouling materials and evaporation processes, scraped surface heat exchangers (SSHEs) are the best choice

Because of the thick nature and high fouling potential of many digestates and sludges, in most cases you need a relatively large surface area to achieve sufficient heat transfer. Because of the limitations off compressors in terms of service fluid temperature, the required heat exchangers and pumps can be exceptionally large indeed. Not only does this increase the capital cost significantly, but as you need larger pumps to push the product through the larger heat exchanger, you also require more energy for operation, so the operational cost benefits begin to reduce.

What type of heat exchanger is best for evaporation?

The simplest heat exchangers are plate heat exchangers (PHEs), which consist of combinations of plates and gaskets through which the product and the heating or cooling medium move, but the highly viscous nature of most effluents and sludges means that tubular or scraped surface heat exchangers are more effective.

The last few years have highlighted the importance of energy costs to businesses, both from an economic and environmental perspective

Tubular heat exchangers consist of different combinations of tubes within tubes, and come in different forms, including those with corrugated tubes. Corrugated tubes increase product turbulence compared to smooth tubes (which are more common), which prevents fouling and improves operating efficiency.

For high fouling materials and evaporation, the HRS Unicus Series of scraped surface heat exchangers (SSHEs) are the best choice.

What are the specific issues with digestate and sludges?

Digestate and sludges often contain suspended solids, which increase the viscosity of the product when evaporated, adding further to the challenges described above. Therefore, if you want to use MVR evaporation, you will need to pre-treat the product to remove as many of these solids as possible, usually via filtration, in order to achieve the necessary rates of heat transfer in the heat exchanger. Again, adding this pre-treatment step adds significant capital costs and reduces potential energy savings during operation.

For high fouling materials and evaporation, the HRS Unicus Series of scraped surface heat exchangers (SSHEs) are the best choice

Digestate also typically contains between 2,000 ppm and 3,000 ppm of ammonia. Pre-treating the product with acid to reduce the pH can neutralise the ammonia, preventing it from evaporating and reducing the risk of damage to the compressor, but once again, this need for acid dosing adds additional cost and operational complexity. You also need to consider the nature of the material and the necessary combination of chemical and physical pre-treatment.

The presence of ammonia creates additional challenges for MVR evaporation of digestate.

As no two sludges are the same, at HRS we always test the material that any client will be working with in order to determine not only the best heat exchanger solution for the evaporation process, but also what pre-treatment may be necessary.

How can you boost energy efficiency without MVR?

Some suppliers like HRS will offer extra levels of customisation, allowing you to specify particular brands of components or controllers

The last few years have highlighted the importance of energy costs to businesses, both from an economic and environmental perspective. Some heat exchanger designs are more energy efficient than others (for example, the use of corrugated tubes improves energy efficiency). In addition, heat exchangers may be able to recover heat from the end of the process and re-use it (known as heat regeneration).

Although potential energy cost savings need to be offset against the capital and running costs of the heat exchanger, your heat exchanger supplier should be able to provide details of the efficiency of all aspects of their equipment and offer heat regeneration where it is desirable.

When choosing an evaporation solution for digestate and sludge, it is important to accurately compare all capital and operating costs.

What else should I consider?

Some suppliers like HRS will also offer extra levels of customisation, allowing you to specify particular brands of components or controllers – for example, to comply with existing factory maintenance contracts or traceability systems. If necessary, you should ask your potential heat exchanger supplier how much influence you, as the client, have on the final design and construction of the system.

If you consider all of these factors from the outset of the procurement process, then you can be sure of getting the right solution for your sludge or digestate evaporation requirements the first time.