Treatment of Liquid Organic Waste

In the treatment of liquid organic waste streams, substances that are dissolved or suspended in water are oxidized at high temperatures (180 - 374 °C) and pressure (40 - 250 bar) in the aqueous phase by adding oxygen. This mainly involves organic substances that are converted to carbon dioxide and water. Some of the substances are converted into lower fatty acids that are readily biodegradable. The process is used to treat liquid waste streams from the following industries: purification, paper and cardboard, food, oil & gas and chemical.

The overall chemical equation for the treatment of sludge is:
C_aH_bO_cN _dP_eS_fX_g + y O₂ → CO₂ + H₂O + NH₄ + PO₄^3- + SO₄^2- + XO + fatty acids + energy

The plant in Apeldoorn, the Netherlands
The treatment plant in Apeldoorn, built from 1991 to 1993, has a yearly capacity of 600,000 m³ liquid waste streams with a dry material content of 5%. Besides communal sewage sludge (75%), sludge from various industrial sewage treatment plants (25%) is also processed.

Steps in the Wet oxidation process*:

1. Acceptance
Liquid waste streams are brought in by tankers and immediately pumped into a 5,000 m³ storage tank. Dehydrated ash cakes are mixed with an internal water recirculation device and also pumped into this tank. The sludge is then pumped from the storage tank to a 1000 m³ batch mixed tank.

2. Supplying the reactor
The liquid stream is pumped into two cutters to reduce coarse particles (permeability 15 mm and 8 mm respectively) and is then pumped into the reactor by worm pumps (number: 6, pump pressure: 35 bar maximum, flow rate per pump: 40 m³/h).

3a. Why is an underground reactor used?

It is a plug flow reactor that allows for maximum conversion without short-circuit flow or residence time distribution, which could reduce the conversions;

Instead of needing high-pressure pumps, the hydrostatic pressure in the column of sludge-water mixture is used;

Heat from the formation helps warming up the reactor during start-up phase (radiant heat);

The pressure on the output side is relatively low, such that a simple pressure regulating valve is sufficient to control the pressure in the reactor;

In the reactor the waste stream is mixed well with the injected gaseous oxygen, giving rise to an optimum reaction.

3b. Oxygen storage
There are two storage tanks for liquid oxygen with a total volume of 84 m³. The liquid oxygen is forced through a heat exchanger using a plunger pump and then injected into the reactor in gaseous form.

4. The reactor
The reactor consists of a system of concentric vertical pipes 1280 metres in length sunk into the ground. The waste stream is pumped downward in the reactor through the inner or downpipe (diameter 200 mm). Pure oxygen gas is added at a depth of 300 m (2,500 kg O₂/h), which triggers oxidation. The oxidized waste stream now converted into ash, returns to the surface via the outer pipe or up-pipe (diameter 300 mm). The average hydraulic residence time is approximately 45 minutes. The temperature and pressure curves have a Gaussian profile: input temperature is 20 °C, bottom temperature is 280 °C and output temperature is approximately 60 °C; initial pressure is 12 bar, bottom pressure is 100 bar and extraction pressure is 18 bar.

The wet oxidation technology

5a. Gas seperation, cooling and treatment
Gases are separated from the ash-water mixture in three stages: in a high-pressure separator (volume 25 m³, operating pressure 18 bar), medium-pressure separator (volume 9.7 m³, pressure 7 bar) and low-pressure separator (volume 19 m³, 1 bar). A tubular cooler is installed between the first two separators with a cooling capacity of 4 MW. In the thermal reheater, the contaminants in the gas (4% carbon monoxide, 1% hydrocarbons) are completely incinerated at 850 °C (gas volume 1,600 m³/h).

5b. Ash separation and dehydration
The ash-water mixture is thickened from 2 to 6% dry matter in a static condenser (volume 1,100 m³). Two membrane filter presses then dehydrate the ash into cakes made up of 55% dry matter. Each press has 140 plates measuring 1500 x 1500 mm, a filling pressure of 8 bar and a membrane pressure of 16 bar. 22,000 tonnes of ash cake are produced annually and shipped in containers to disposal sites. The ash contains approximately 38% aluminium phosphate, 16% silicates, 12% quartz, 9% calcite, 5% Portlandite, 4% feldspars and 16% remaining solids.

5c. Process water treatment
The remaining water is pumped into a 5,000 m³ storage tank. From here it flows out at a constant rate for biological purification. This process consists of denitrification (volume 4,000 m³, 4 mixers) and aerobic nitrification (volume 16,400 m³, 4 blowers of 250 kW each, 1,700 aeration elements). The chemical oxygen demand (COD) is reduced by 93%, the biological oxygen demand (BOD) by 99.8% (effluent <10 mg/l), Kj-N reduction 97.5%, and
ammonium (NH_4-N) by 99.9% (effluent <1.5 mg/l).

5d. Acid cleaning system
During the process of wet oxidation, anhydrous calcium sulphate
(Ca SO₄ 0H₂ 0 aq) is also produced.
Some of this collects on the reactor wall. This scalelike deposit is removed from time to time using nitric acid (15%). An acidification tank (45 m³, 50% acid) and two acid tanks (100 m³ each, 15% acid) have been installed for this purpose.

Note *): Based on the Apeldoorn plant experiences with sludge treatment period 1992 to 2004