Advanced anaerobic digestion configurations

Thermal hydrolysis can be integrated into the sludge line in four configurations (pre-treatment, inter-treatment, post-treatment, or WAS-only), with different CAPEX (investment in digestion and thermal treatment) and OPEX (net biogas generation and hygienisation).

Pre-treatment minimises digestion CAPEX, while inter-treatment improves the energy balance when digestion is oversized. Secondary-sludge-only hydrolysis combines both advantages without achieving hygienisation, and post-treatment prioritises reducing the biosolids volume.

The optimum configuration depends on the balance between these factors, energy and biosolids management costs, and the specific regulatory requirements in each case.

The four sludge line configurations with thermal hydrolysis

Thermal hydrolysis can be integrated into the sludge line in different ways, depending on its location in relation to anaerobic digestion and on the sludge it treats. Each of the four main configurations involves a different trade-off between capital expenditure (CAPEX) and operating costs (OPEX).

1

Pre-treatment

Pre-treatment diagram: thermal hydrolysis of fresh mixed sludge before anaerobic digestion

In this configuration, thermal hydrolysis treats all primary and secondary sludge before it enters digestion. It is the most common scheme and enables a very significant reduction in digestion volume and its associated CAPEX, in exchange for a larger thermal process that treats 100% of the fresh sludge. It is the reference configuration for new plants where the priority is to minimise investment in digestion.

2

Inter-treatment

Inter-treatment diagram: thermal hydrolysis between anaerobic pre-digestion and post-digestion

Thermal hydrolysis is located between pre-digestion and post-digestion. As some of the volatile solids are removed in pre-digestion, the flow rate to thermal treatment is reduced and, therefore, so are its size, cost, and energy consumption. In post-digestion, the conversion of solubilised matter into biogas is maximised, achieving better overall energy efficiency. The drawback of this configuration is that it requires a larger digestion volume, as it has two biological stages. This makes it especially interesting when existing digestion is oversized or when OPEX resulting from the energy balance is more critical than the CAPEX of digestion.

3

Post-treatment

Post-treatment diagram: thermal hydrolysis after anaerobic digestion to improve biosolids dewatering

Thermal hydrolysis is placed after digestion to minimise the biosolids volume via dryness increase, since hydrolysed sludge dewaters better than digested sludge. The post-dewatering sidestream, with a high soluble COD fraction, is recycled back to the digester to be converted into biogas. A less mature configuration, it may be suitable when minimising the biosolids management cost is the main economic driver.

4

WAS-only

Diagram of thermal hydrolysis of WAS-only: hydrolysed biological sludge mixed with non-hydrolysed primary sludge

Another option is to hydrolyse only the secondary or biological sludge, which is fed to digestion after being mixed with non-hydrolysed primary sludge. By treating only part of the fresh sludge, thermal hydrolysis is significantly smaller and requires less energy than in pre-treatment. The energy balance is favorable, because primary sludge is highly biodegradable per se, and the main disadvantage is that, unlike the other schemes, it does not guarantee pathogen removal. It can therefore be an interesting alternative when hygienisation is not a regulatory requirement.

Advanced anaerobic digestion configurations comparison

The main technical parameters of the four advanced anaerobic digestion schemes with thermal hydrolysis are compared below. There is no universally better configuration; rather, the best option will be specific to the energy costs and biosolids management costs, the existing installed capacity, and the regulatory requirements of each particular case.

For this reason, the selection of the optimum solution is addressed through a customized techno-economic analysis. If you would like a feasibility and profitability study on how these configurations can be implemented using the Polanco Process for continuous thermal hydrolysis, send us a few details about your plant and we will deliver it to you in less than 2 weeks.

Technical comparison of the four thermal hydrolysis configurations:
Conventional anaerobic digestion (1)
Pre-treatment 		(2)
Inter-treatment 		(3)
Post-treatment 		(4)
WAS-only
Thermal hydrolysis feed - Fresh mixed sludge Digested mixed sludge Digested mixed sludge + filtrates Fresh secondary sludge
CAPEX Anaerobic digestion volume 1001 < 50 100 100 < 55
Thermal hydrolysis capacity - 1001 < 66 < 66 < 45
OPEX Energy efficiency Low Medium Very high High High
Biosolids management cost High Low Low Very low Low
Hygienised biosolids No Yes Yes Yes No
Determining factor - Minimisation of digestion CAPEX Optimisation of energy OPEX Reduction of biosolids management cost Hygienisation is not a requirement
1 Comparison base 100

Frequently asked questions about thermal hydrolysis configurations

What is the most common thermal hydrolysis configuration?

Pre-treatment of sludge before anaerobic digestion is the most widespread configuration because, by minimising the digestion volume and associated CAPEX, it is especially attractive for new plants.

Which configuration offers the best overall energy balance?

Inter-treatment, where thermal hydrolysis is located between pre-digestion and post-digestion, hydrolyses the digested sludge and maximises energy recovery in the form of biogas.

Which configuration is most suitable when the existing digestion capacity is oversized?

Inter-treatment is the preferred option when there is overcapacity and no additional investment in digestion is required, as it prioritises energy OPEX.

When does it make sense to apply thermal hydrolysis as post-treatment?

When the main objective of the project is to reduce the volume and management cost of biosolids, since in this configuration the improvement in dewaterability is the main benefit.

In which cases can thermal hydrolysis of WAS-only be chosen?

When a compact and energy-efficient thermal system is sought and hygienisation is not a regulatory requirement. By treating only the biological sludge, the size of the installation and its energy demand are reduced, but at the cost of not achieving biosolids hygienisation.

Is there a thermal hydrolysis configuration that is better than the others?

No. The optimum configuration depends on the balance between CAPEX and OPEX, energy and biosolids management costs, and regulatory requirements, so it must be identified on a case-by-case basis through a techno-economic feasibility analysis.

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