Wetlands As Evaporation And Treatment System For Produced Water
Authors: de Leon N. , Camacho F. , Ceci N. , Velasquez J. , Colombo P. 
Published: December 2007

In oil fields where high volumes of produced water have to be managed for its conditioning and further disposal following environmental regulation, the production cost is considerably impacted by the problem of water management. Present study shows experimental results obtained from greenhouse tests during 1998 and 1999, carried out using produced waters with chloride content ranging from 3,000–8,000 ppm, and phenol content from 2.0–7.0 ppm. The major objective of the study was to determine the capacity of a "constructed wetland" for removing organic compounds present in water and the impact of plant evapotranspiration rate on whole system water balance. Treatments applied to effluents consisted in simulating various constructed wetland systems constituted by halophyte plants species which resist low-moderate levels of salinity (Eleocharis mutata, Typha domingensis, Echinochloa cruz-galli). Produced water was continuously fed to wetlands for approximately five months. Water residence time in Constructed Wetland was between two and ten days. Halophyte plants endured wetland operating conditions. A 98% reduction is reported in phenol and sulphur content, a 70% reduction is reported in BOD5, a 50% in COD, and a 20% in chlorides content. No toxicity effect was detected in effluent out put water toxicity bioassays. A 70% of the water fed to wetlands was evaporated during dry season at a residence time of about three days.

For produced water with high chloride content, wetlands are proposed to be used as a natural evaporating system, in which halophyte plants uptake water and evapo-transpire it, reducing water volume and the associated costs by effluents reinjection. For produced water with organic compounds content, wetlands are proposed to be used as treatment system. Wetlands could be designed for specific desired results, depending on produced water characteristics and prevalent environmental conditions.

The biological process (Constructed Wetlands) used for removal of organic compounds implies the use of state of the art green technologies, which are easy to operate at a very low investment cost.

Introduction

Primary treatment applied to effluents of most produced water stations in Barinas and Apure states basically consists in removing crude contained in waters, and cooling them.

Below, a brief description of processes being performed in the primary treatment system of produced waters in Barinas State stations, specifically in Silvan Station is presented.

Crude oil from production wells is fed to a washing tank, where decantation of most of the crude oil with less than 1% of water and sediment takes place. Two streams are produced in the washing tank discharge; one is made up of crude, which is sent to a refinery, the other is made up of water, which is conditioned to be delivered to the environment.

Before feeding crude into the washing tank, a demulsifying agent is applied to favor crude dehydration, aimed to meet quality specifications required by refining processes. Also, a clarifying agent is used to promote flocculation of emulsified crude found in produced waters, as well as to fulfill current environmental regulations.

Effluents from washing tank are fed to an API separator, where part of free crude in these waters is decanted. Then, effluents are sent to a system of three cooling-pools connected in series, in order to decrease the temperature of water to be discharged to the environment. Actually is prevered the use of flotation cell as a secondary treatment to favor emulsioned oil remove.

A new option for treating produced waters as a third treatment, consists in using constructed wetland systems constituted by halophyte plant species (Settle, 1998).