AWZI Houtrust

1. Remove large solid materials
The Houtrust waste water treatment plant (WWTP) was originally built in 1967. It was partially renovated in 1988 and 1995 to meet the (environmental) demands of that time. Even so, around 2000 the treatment plant was showing its age and was no longer capable of effectively treating the waste water flow from the homes and companies in The Hague. Moreover, the plant could not meet the more stringent European requirements for nitrogen removal. This was sufficient reason to totally refurbish this waste water treatment plant. In 2007 and 2008, the Houtrust WWTP was drastically renovated. With the current treatment capacity of 430,000 pollution equivalents (p.e.) the plant is able to treat the waste water from the residents and companies of The Hague and surrounding areas. On a site that covers 5 hectares, the Houtrust WWTP can treat on average almost 79,000 cubic metres of waste water a day, in other words the capacity of more than 2,600 trucks.

The biological waste water treatment process is as ingenious as it is simple: we employ billions of micro-organisms that do the actual work for us, they literally consume the waste material in the water. In fact this is exactly what happens in nature, but here in a controlled way and on a large scale.

The entire treatment process is fully automated. The operator can log in remotely and, if required, intervene. Pumping stations transport the waste water from the municipalities to the WWTP.

Immediately on arrival, remove the large solid materials (paper, plastic, wood, textiles) from the waste water. After being cleaned, this waste material is stored in containers and transported to a waste incineration plant outside of the WWTP.

2. Pre-sedimentation tanks
In the pre-sedimentation tanks, the waste water is brought to rest, floating material (oils and fats) is skimmed off and heavier, non-soluble particles (sediment) settle to the bottom to form a layer of sludge. This so-called ‘primary sludge’ is transported to the sludge treament plant and does not return to the water treatment process.

3. Activated sludge
The activated sludge system is the heart of the biological treatment process. This system contains large numbers of micro-organisms – ‘activated sludge’ – that alternately with and without aeration are continually in contact with the waste water.

4. Biological treatement
These steps remove approximately 80% of the nitrogen and phosphate compounds and more than 90% of the organic contamination.

5. Post-sedimentation
After this biological process, the sludge/water mix comes to rest in the post-sedimentation tanks, where the activated sludge – containing the micro-organisms that consumed most of the contamination in the waste water – settles to the bottom and is partly returned to the biological tanks (3) where it is used once more. It is extremely important that the waste water remains sufficiently long in the post-sedimentation tanks, to allow the activated sludge sufficient time to settle to the bottom. The treated waste water (effluent) flows to the effluent pumping station to be transported further.

6. Primary sludge thickening
The primary sludge (6) from the pre-sedimentation tanks (Sludge Flow 1) and the surplus activated sludge from the post-sedimentation tanks (Sludge Flow 2) are thickened, in order to remove as much water as possible. The sludge then goes to the digestion tank.

7. Fermentation process
In the digestion tank the sludge partially decomposes at a temperature of 35ºC. During this fermentation process, biogas is released.

8. Generating energy
The produced biogas is used to operate the gas engines that generate heat and electricity for the treatment plant. The generated energy supplies over 40% of the plant’s own energy needs.

9. Dewatering sludge and transport
After the sludge has fermented sufficiently, the remaining mass is dewatered in the sludge dewatering centrifuges, which results in a concentrated waste product. Trucks – around thirteen full trucks each week – transport this dewatered sludge to the HVC sludge incineration plant in Dordrecht, which is jointly owned by various water boards and municipalities.

10. Air treatment
WWTP Houtrust is equipped with 3 air treatment installations, one for the pre-treatment area, one for the sludge treatment area and one for the sludge loading area. The first one is a gas scrubber, where air is brought into contact with the air, while chemicals are added. The second one is a biological filter and the air treatment of the sludge loading area comprises of 4 active carbon filters.

The cleaned air, in total 80.000 m3 per hour, is finally discharged through a stack, under stringent emission requirements.

11. Energy
In order to have the biological treatment process perform properly, oxygen will have to be added to the water. This is realized by blowing air into the aeration tanks. The blowers that take care of the air supply, are large energy consumers in the WWTP. On top of that, the treated waste water, together with the treated waste water from WWTP Harnaschpolder, has to be pumped through a 2.5 kilometer long discharge main, far from the coast into the Northsea.

The required electricity for the treatment of the incoming waste water and the transport of the treated waste water to the Northsea, is partly supplied by the energy company and is partly generated by our own generators. The methane gas, used for the generators, is produced in the digesters using the sludge streams from the WWTP. The gas driven generators have sufficient capacity to still fully treat all incoming waste water in 90% of the time, in case of an interruption of the electricity supply by the energy company.