AWZI Harnaschpolder

The Harnaschpolder waste water treatment plant (WWTP) has a treatment capacity of 1.3 million pollution equivalents (p.e.), which makes it the largest in the Netherlands and one of the largest in Europe.

Process explanation

1. Remove large solid materials
The Harnaschpolder waste water treatment plant (WWTP) has a treatment capacity of 1.3 million pollution equivalents (p.e.), which makes it the largest in the Netherlands and one of the largest in Europe. On a site that covers no less than 25 hectares, Delfluent Services treats the waste water from more than 1 million inhabitants and around forty thousand companies in the region of The Hague. On average, the WWTP can treat 255,000 cubic metres of waste water a day, in other words the capacity of more than 8,000 trucks. Construction of WWTP Harnaschpolder started early 2004. By the end of 2006 the WWTP was taken into operation.

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 naturally, but then 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, etc.) from the waste water. After being cleaned, this separated waste material is stored in containers and later transported to a waste incineration plant outside of the WWTP.

2. Removal of oils and fats
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. Biological treatment
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. This step removes more than 85% of the nitrogen and phosphate compounds and more than 90% of the organic contamination.

4. Effluent to WWTP Houtrust
After the biological process, the sludge/water mix comes to rest in post-sedimentation tanks, where the activated sludge – containing the micro-organisms that have consumed significant amounts of the contamination in the waste water – settles to the bottom and is partly returned to the aeration tanks 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. It still has a long way to go: first it flows through two effluent pipelines, each more than ten kilometres long to the Houtrust WWTP inThe Hague and then, together with the effluent from Houtrust, it is pumped through aother 2.5-kilometre long pipeline to end up in the North Sea, far from the coast. 

5. Primary sludge
The primary sludge from the pre-sedimentation tanks and the surplus activated sludge from the post-sedimentation tanks are thickened, in order to remove as much water as possible.

6. Fermentation process
The sludge then goes to the digestion tanks, where it partially decomposes by micro-organisms / micro-organisms at a temperature of 35ºC. During this fermentation process, biogas is released.

7. 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 50% of the plant’s own energy needs.

8. 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 eight full trucks each working day – transport this dewatered sludge to the HVC sludge incineration plant in Dordrecht, which is jointly owned by various water boards and municipalities.

9. Biological air treatment
Air from all buildings and tanks where odour from the waste water is available, is extracted in order to create an under pressure to prevent odour emissions.

In total 60.000 m3 air per hour, containing several penetrating odour substances, is extracted from the buildings and tanks and is transported to the biological air treatment system. In the first biological treatment step (two parallel filters) hydrogen-sulfide is removed. In the next biological treatment step (four parallel filters) the remaining odour substances are removed. The clean air is subsequently discharged through the chimney.

The first compartment of each of the eight aeration tanks, where the waste water is treated, consists of a non-aerated area (anaerobic tank). The air above the water level of these anaerobic tanks is free of hydrogen-sulfide but contains several other odour substances.

From each anaerobic tank a relatively small quantity of air is extracted to secure under pressure to prevent odour emission. This extracted air is mixed with the inlet air of the aeration blowers and is fed to the aeration tanks. The bacteria, which are present in the aeration tanks, not only clean the waste water, but also remove the odour substances originating from the anaerobic tanks. The exhaust air of the aeration tanks, now free from other odour substances, is discharged through the chimney, together with the cleaned air from the biological air treatment system. The air discharged by the chimney takes place under under stringent emission requirements.

10. 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 biological 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 has to be pumped over a relative long distance, before it can be discharged in the Northsea.

The required electricity for the treatment of the incoming waste water and the transport of the treated waste water to Houtrust, 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. Besides the 3 gas generators, Harnaschpolder is equipped with 2 diesel driven generators. Each generator has a capacity of 1.330 kW. In case of an interruption of the electricity supply by the energy company, we can still fully treat all incoming waste water in 90% of the time.