Polanco Process diagram: continuous thermal hydrolysis step by step

The Polanco Process subjects sludge to four continuous stages:

  • Feeding and homogenisation of pre-dewatered sludge at 14-16%DS, preheated with low-pressure steam recovered from the process;
  • Preheating with medium-pressure steam and evacuation of non-condensable gases from the degasifier, which operates at 0.5-1.5 barg;
  • Thermal hydrolysis in the reactor at 140-160°C with hydraulic retention times (HRT) of 15-30 minutes, before the first flash or steam explosion occurs;
  • Second flash in the atmospheric tank (0.1-0.4 barg), which recovers low-pressure steam and delivers the hydrolysed sludge to anaerobic digestion.

The Polanco Process step-by-step

The operation of the Polanco Process for continuous thermal hydrolysis is shown step by step:

1

Feeding and homogenisation

Stage 1 diagram of the Polanco Process: feeding and homogenisation of pre-dewatered sludge at 14-16%DS with recirculation and preheating with low-pressure steam
Stage 1: fresh sludge (14-16%DS) is mixed with the recirculation stream and preheated with low-pressure steam recovered from the process before entering the mixing tank.

The fresh sludge, pre-dewatered to 14-16%DS and at ambient temperature, is mixed with a recirculation stream and preheated with the low-pressure steam recovered from the process before being fed to the mixing tank.

2

Degassing

Stage 2 diagram of the Polanco Process: degasifier at 0.5-1.5 barg with medium-pressure in-line steam injector for non-condensable gas evacuation
Stage 2: the sludge passes through the in-line steam injector (medium pressure) and feeds the degasifier (0.5-1.5 barg), which removes the non-condensable gases from the process.

The homogenised sludge passes through an in-line steam injector, where it is preheated with medium-pressure steam, and feeds the degasifier, which operates at a pressure of 0.5-1.5 barg and is designed to evacuate the non-condensable gases.

3

Thermal hydrolysis

Stage 3 diagram of the Polanco Process: continuous thermal hydrolysis reactor at 140-160°C with HRT of 15-30 minutes and first medium-pressure steam flash
Stage 3: thermal hydrolysis in the reactor at 140-160°C with HRT of 15-30 minutes. The first flash separates medium-pressure steam (dome) from the hydrolysed sludge (bottom).

The degasified sludge flows through an in-line steam injector and reaches its highest temperature thanks to high-pressure boiler steam. Here, the first flash or steam explosion partially vaporises the sludge, and the two-phase flow is separated in the reactor: medium-pressure steam at the top, and the liquid phase, hydrolysed at 140-160ºC and with hydraulic retention times (HRT) of 15-30 minutes, at the bottom.

4

Flash or steam explosion

Stage 4 diagram of the Polanco Process: second flash in atmospheric tank at 0.1-0.4 barg with low-pressure steam recovery and hydrolysed sludge outlet to anaerobic digestion
Stage 4: second flash in the atmospheric tank (0.1-0.4 barg). Low-pressure steam is recovered back to stage 1; the cooled and diluted hydrolysed sludge feeds anaerobic digestion at 35-40°C and 8-10%DS.

The thermally hydrolysed sludge leaves the reactor and proceeds to the second flash, where it is depressurised and split into two streams separated by the atmospheric flash tank (0.1 – 0.4 barg). Low-pressure steam is recovered at the top, while the hydrolysed sludge leaves at the bottom and, after cooling and dilution, feeds anaerobic digestion at 35-40ºC and 8-10%DS.

Frequently asked questions about the stages of the Polanco Process

How many stages does the Polanco Process have, and in what order are they carried out?

The Polanco Process has four stages that operate continuously: sludge feeding and homogenisation, non-condensable gas degassing, thermal hydrolysis in the reactor and flash or steam explosion. These stages use the process steam generated to maximise the process's energy recovery.

What conditions must the sludge meet before entering the Polanco Process?

The sludge must be pre-dewatered to a concentration of between 14 and 16%DS and be at ambient temperature. Before entering the mixing tank, it is preheated with the low-pressure steam recovered from the process, optimising the overall energy balance.

What is the degasifier and why is it necessary in thermal hydrolysis?

The degasifier, which operates at 0.5-1.5 barg, is designed to evacuate the non-condensable gases generated during the thermal process. If they are not removed from the process, these gases accumulate and reduce hydrolysis efficiency.

What is flash or steam explosion in thermal hydrolysis?

It is the sudden depressurization of superheated sludge, which generates a two-phase stream that splits into steam, which is recovered for preheating, and hydrolysed liquid. The Polanco Process applies a double flash mechanism: the first, before the reactor, generates medium-pressure steam; the second, before the atmospheric flash tank (0.1-0.4 barg), recovers low-pressure steam.

At what temperature and concentration does the sludge leave after the complete process?

The hydrolysed sludge leaves the Polanco Process at 102-106ºC and with a concentration of 12-14%DS. After a subsequent cooling and dilution stage, it is fed to anaerobic digestion at 35-40°C and a concentration of 8-10%DS. Under these conditions, its greater biodegradability makes it possible to increase biogas production by up to +35% (verified at Copero WWTP, Seville, since 2020).

How does the Polanco Process recover energy between stages?

Through a cascade steam recovery system: the low-pressure steam generated in the second flash preheats the sludge at the inlet (Step 1); the medium-pressure steam produced in the first flash preheats the sludge before the degasifier (Step 2). This reduces the process's specific energy consumption to less than 0.15 kg steam/kg sludge.

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