Summary and Conclusions

Optimering og dokumentation af biologisk renseanlæg på Østre Gasværk

Oxygen and Sand Used for Treatment at Gasworks Sites

A biological treatment plant at Østre Gasværk uses a new method for on-site treatment of contaminated groundwater. Contaminated water is treated in sand filters after infusion of pure oxygen. The method has a documented positive effect on removal of BTEX, naphthalene, phenols, PAH and partly of cyanide, and can therefore be used for treatment of tar, oil and petrol contamination. The contamination is decomposed and therefore not transferred to other media such as air or active carbon. The method is robust against changes in contamination concentrations and stop-downs. The cost of water treatment using this method is DKK 7-17 per m³and is comparable with other remediation methods for treatment of contaminated water from gasworks sites.

Background and Objectivestrong

The treatment of groundwater from gasworks has turned out to be difficult, since the water contains a complex mixture of organic and inorganic components. The development of plants for treatment of this type of water is therefore of enormous interest.

In 2002 the Environmental Protection Agency of Copenhagen (EPAC) established a biological water treatment facility for treatment of drainage and groundwater at Østre Gasværk. The former gasworks operations on the site mean that drainage and groundwater is contaminated with, among other, tar components, cyanide and ammonium.

The overall objective of the technology project has been to improve existing knowledge in order to create a more general and solid base for design and scoping of new facilities for biological treatment of contaminated groundwater.

This has been done partly by optimising the operational procedure at EPAC’s facility at Østre Gasværk with respect to oxygen consumption and retention times, and partly by documenting the optimised operation by establishing mass balances for contamination components and making toxicity tests. Furthermore, the importance of stop-downs has been investigated. The optimisation procedure has attempted to improve the conditions for removal of ammonium and cyanide.

The Investigation

ECAP’s biological water treatment facility is installed in a container in the southeast corner of Østre Gasværk and can treat 6-10 m³/h drainage and groundwater. The facility is designed with two parallel treatment process lines, each consisting of two sand filters each with an effective volume of 1.5 m³. Before each filter, the water is infused with 95-100 % pure oxygen from an Oxymat unit.

In connection with the technology project, a series of tests were conducted in the period July - October 2005 in the biological treatment facility. During the tests, the oxygen and retention time in one of the process lines varied between 3.237 and 11.653 mg oxygen/min and 0.5 to 5 hours. The other process line was used as reference.

During the test series, water samples were taken from the common inlet and outlet from each process line. The water samples was analysed for a series of substances, e.g. PAH, BTEX, phenols, ammonium and cyanide, in order to investigate the decomposition of these substances in the facility. The toxicity of the water was investigated using biotox tests.

Main Conclusions

The biological water treatment facility at the Østre Gasværk site is under normal operation conditions capable of decomposing tar components such as BTEX, PAH and phenols in the order of 95-99 %. Please, refer to the table below.

On the basis of the test series it is concluded that

Decomposition is dependent on sufficient oxygen supply. If the oxygen supply is reduced, there is first a reduction in phenol decomposition, then PAH and finally BTEX is removed. An optimal decomposition requires supply of 3.5 - 4.5 l oxygen/m to each filter at a total performance of 2 - 2.5 m³/h, corresponding to infusing a total of 550 mg oxygen per litre total treated water in the facility.
Decomposition lessens dependence on retention times within the test series variations of 0.5 - 5.5 hours.
Decomposition of cyanide and phenol is difficult to start up after prolonged stop-downs (> 1 month).
A clear reduction in inherent toxicity takes place during passage through the biological water treatment facility. The inlet water toxicity is reduced by a factor 2.2.

Project Results

Biomass in the treatment facility quickly adapts to conditions and the contamination is decomposed efficiently, although the total content of tar components has varied between 1-18 μg/l at the inlet.

Analysis parameters	Østre Gasværk water treatment plant		De- composition	Quality require- ments for water areas and outlet to sea /11/, * ground- water criterion /12/
Analysis parameters	Inlet μg/l	Outlet μg/l	%
Benzene	24-5600	0,06-5	99	2
Toluene	130-3500	0.2-3.9	97-100	10
Ethyl benzene	<0,2-860	0.2-5.1	100	10
Xylenes	220-2800	0,79-90	97-100	10
PAHs sum	180-4700	8.6-69	99	* 0.2
Phenol	6-870	0.48-4.8	92-99	1.000
Cyanide	200-660	50-210	30-80	* 50

Table Decomposition of tar components in biological sand filter

Ammonium is not removed in the facility and therefore no nitrification has taken place. This could be because pH is in the interval 6.5-7, which is at the lowest of what nitrification prefers. Another cause could be that toxicity in the remedial water restricts the nitrifiers.

Cyanide is typically problematic at gasworks sites. In the drainage and groundwater at Østre Gasværk, there has been found no free cyanide and the facility only partly transforms bound cyanide, typically 30-60 % of the inlet’s cyanide contents.

It cannot be excluded that the lack of cyanide transformation is due to the non-existing nitrification. Nothing indicates that the concentrations of cyanide have an inhibiting effect on decomposition rates.

An evaluation of the decomposition rates in the treatment facility shows that contamination is decomposed and not only transferred to the air phase. Decomposition is not complete. Up to 1 gram of substituted phenols per hour is formed in the facility, as decomposition products from other aromatic compounds and poly-aromatic compounds.

The biological water treatment facility at Østre Gasværk has a good treatment effect for tar components. As the table above shows, the discharge water from the water treatment facility at Østre Gasværk complies with recipient quality requirements for most substances, with the assumption of a 300 times dilution. Even sum concentrations of PAH have been brought down to comply with quality requirements. The most problematic requirement in terms of discharge to recipient is that of 0.001 µg/l to single PAH and ammonium which is not decomposed in the water treatment facility and is at a level of around 15-20 μg/l.

If the biological water treatment facility is optimised to decompose ammonium, e.g. by grafting and pH adjustment, it would be optimal for gasworks since the traditional methods of active carbon filtering and stripping cannot remove both tar components and cyanide/ammonium in the water phase.

The price for treatment of groundwater in the biological water treatment facility is 7-17 DKK per m³, excl. discharge fee for sewers, and is comparable with other remedial methods for treatment of water contaminated by tar components.