DDT and CHC groundwater treatment
A groundwater treatment unit for the remediation of DDT and CHC damage on an industrial site - 9600 m³ capacity / day
Introduction
Over a long period of time DDT concentrations of up to approx. 3 µg/l (ppb) together with iron compounds at concentrations of 2-10 mg/l (ppm) and chlororganica of approx. 4.2 mg/l (ppm) flowed from a production facility in North Italy into Lake Maggiore. The pollutants, in particular DDT and its derivatives settled and concentrated in the mud on the bed of the lake.
They reach the human food chain via the plankton particles released from the mud. These are consumed by the lake's fish where the DDT, DDD and DDE remain stored in the fish's fatty tissues. This was verified for the first time in 1996 by research on sea-fish. In order to avoid a build up of DDT in humans, commercial fishing in Lake Maggiore has been forbidden until further notice.
The danger DDT presents to humans exists because of it's affinity for human genetic material, whereby gene damage is probable.
Objective
By using a groundwater catchment system consisting of 4 wells, it should be possible to collect the pollutants at their point of origin. Because of the large amounts of groundwater arising and the high porosity of the water-bearing layer the total mechanical pumping capacity is approx. 400 m³/h.
Before the water is returned to the ground, the treatment plant must reduce concentrations in the groundwater fed through it to the following extremely low target levels:
* DDT to 0.05 µg/l (ppb), Chlororganica to 50 µg/l (ppb) and
* Iron compounds to 0.05 mg/l (ppm)
(In operation, concentrations were clearly under the limits. In some cases under the detection limit for the individual substance.)
The hydraulic boundary conditions were defined as followed by the authorities:
* Total extraction 350 m³/h,
* Catchment at 4 points (100 : 100 : 100 : 50) m³/h
* Direct return.
DDT Elimination on an industrial scale
No comparable remediation projects are known of world-wide, and thus there was no comparable process data available. The development of the final configuration began on a laboratory scale with the inclusion of the known chemical properties of DDT.
After a short period of time, tests were carried out on a pilot-scale plant in order to be able to approach an industrial-scale operating process. With a scaled-down flow of 5 m3/h both the interaction of the groundwater with operation of the pilot plant in a dynamic system, and the results of laboratory batch testing were analysed to demonstrate the suitability of the processes planned.
A substantial part of the work during the development phase was in the determination and synchronisation of response times for the individual process modules. Special complications were presented by the precipitation dependent fluctuations of the contaminant spectrum.
Despite following the usual DIN regulations, several laboratories conducting analysis work in parallel were unable to provide consistent analyses at the beginning. Working together with these laboratories we were able to develop a reliable procedure for the quantitative determination of DDT traces in water samples with sensitivity down to 30 ng/l.
Treatment
The treatment of the groundwater takes place in 3 stages with increasing elimination of the pollutants relative to their initial concentrations. The system combines a broad spectrum of procedures from process engineering by using 3 main modules, chemical process (flocculation/coagulation), physical process (stripping/desorption), and physio-chemical process (filtration, contact catalysis).
1a. Flocculation / Coagulation
90 % iron elimination
90 % DDT elimination
* Deferrisation module in the acqueous phase
1b. Multi-layer filter
99% elimination of Fe
* Catalytic deferrisation in the solids filter
2. Desorption/ Adsorption
99% elimination of CHCs
* Water-air stripping and
* Adsorption of the loaded strip-air onto activated carbon
3. Activated carbon filtration
99% elimination of DDT
* Activated carbon adsorption
Based upon experiences in the pilot tests at 5 m3/h, a multiple-street structure was suggested in order to be able to manage the scale-up to 350 m 3/h as quickly as possible and with the minimum of risk. The industrial scale plant was realised in 2 stages:
* First stage of development with two parallel streets, each of 100 m³/h (equivalent to a scaling factor of approx. 20)
* Second stage of development with a third street up to 400 m³/h
Without the possibility of intermediary buffering of the treated water, the required target values could be achieved directly from start-up. Owing to efficient project management, the short deadlines set by the customer could be kept. PRANTNER not only took on process development, but also the complete construction, design, assembly and operation of the system. The plant runs fully automatically and is equipped with PLC and process visualisation. Installed telemetry systems allow operating parameters to be called-up from a remote location.
Authors:
Dr. Stefan Koppe, Jürgen Prantner, Prantner GmbH Verfahrenstechnik, Reutlingen
Published in:
TerraTech 6 Nov./Dez. 1998 (Vereinigte Fachverlage GmbH, Mainz)
CHEManager 1/99 (GIT Verlag, Darmstadt)
Chemie Technik, Nr. 7 1999 (Hüthig GmbH)
"In 2001, the unit was expanded to a purification capacity of 850 m³/h by the Prantner GmbH Verfahrenstechnik."