Thermal hydrolysis configurations

APRIL 18, 2024

Although Thermal Hydrolysis (TH) technology was originally a pre-treatment to Anaerobic Digestion (AD), it can be placed in different locations within the sludge line depending on the specific characteristics of each plant.

The main configurations, with their advantages and disadvantages, are:

Pre-treatment

In most cases, TH is located before the AD, that is fed and easy-to-digest hydrolyzed sludge in pre-treatment. 

The main advantages of this configuration are smaller digestion volume (half or even a third of the standard design digestion volume), a biogas yield increase, a biosolid volume reduction and its hygienization.

The main disadvantage is that, as all the sludge is hydrolyzed, the TH is relatively large (and expensive) and requires more energy (the waste heat from the hot exhaust gases of the engines may not generate all the required steam, having to generate part via the biogas burner).

A variant of this pre-treatment configuration is one in which only the secondary sludge (WAS) is hydrolyzed, and mixed downstream with the raw primary sludge.

This configuration reduces the TH size (and cost), improves the energy balance by hydrolyzing less material and also reduces the digestion volume required by a conventional design. But it does so at the cost of not hygienizing the biosolid (this can be acceptable under applicable legislation).

Inter-treatment

TH can be located between pre- and post-anaerobic digestion in inter-treatment.

This configuration presents a better energy balance because it achieves the greatest biogas yield increase with the lowest steam consumption (the sludge has already been digested, so the volume to be hydrolyzed is smaller and the hot exhaust gases waste heat is usually enough to generate all the TH steam). Furthermore, due to this prior digestion, the TH is relatively small (and cheap) and achieves a greater biosolid volume reduction as well as its hygienization.

The main drawback is a larger digestion volume. If digestion is oversized, not uncommon, this drawback is not such and this configuration may be the most attractive. In fact, teCH4+ has several references of commercial plants operating with this configuration.

Post-treatment

In some specific cases, the TH can be located after AD, directly feeding the biosolid dewatering in post-treatment. 

The main advantage is a greater biosolid volume reduction, since the hydrolyzed sludge can be further dewatered. The biosolid remains a pathogen-free, hygienized product.

To take advantage of the TH-generated soluble COD, the dewatering liquid is returned to the digester, although it will be difficult to match other configurations´ biogas yield.

To maximize dryness, the hydrolyzed sludge heat is often used for hot dewatering, presenting its own dewatering adjustment and odor management challenges.

Conclusion

TH technology can be implemented in different configurations, with distinct advantages and disadvantages. To identify the optimal scheme, the most sensible thing to do is to carry out a rigorous feasibility study taking into account the technical and economic specifics of the WWTP.

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