PROJECT DESCRIPTION

Introduction

The Goreangab Water Reclamation Plant is internationally renowned as the first Plant in the world to reclaim domestic sewage for drinking water purposes. After 32 years it continues to be the only Plant in the world to do so. The latest available water treatment technology was utilized in the design of the new 21 MI/d Plant, which was completed in 2002.
Namibia is the most arid country in Southern Africa and continuously faces serious water challenges.
The initial Goreangab Water Reclamation Plant was built by the City of Windhoek in 1967 to reclaim water directly from domestic sewage effluent as a supplement to Windhoek’s very scarce raw water resources.
The Plant can be fed from two sources, these being the Gammams Sewage Treatment Plant and the Goreangab Dam. The Plant can be split into two streams, with one stream used for effluent from the sewage treatment plant and the other used for the treatment of Goreangab Dam water. Alternatively the raw waters can be blended and treated as a single stream. Due to pollution in the catchment area of the dam, the quality of the water had deteriorated to such an extent that conventional treatment methods could no longer be applied. The two sources had to be combined and treated in a single extended and upgraded new Goreangab Water Reclamation Plant.
During the early nineties, the need arose to augment the Plant significantly and to upgrade its treatment train with the latest technology to address the most recent health concerns.
GFJ formed a consortium with Fichtner in Germany and MultiConsult in Namibia and was awarded the technical responsibility for this challenging project.
 

Purpose

The purpose of the project was to exploit the maximum reclaimable and fresh water from sources available to Windhoek and by doing so, contribute to meeting the increasing demand for potable water and add to the security of supply to the City of Windhoek.

Approach

Since the commissioning of the first reclamation Plant in 1967 up to the middle 90’s when this project was started, a huge amount of new information on various aspects of water treatment became known. Experience obtained from the old Plant, information from pilot plant studies and knowledge of current technologies had to be integrated into a process design which eventually lead to a reclamation Plant producing water of a final quality which is sustainably fit for human consumption.
The new treatment Plant was designed using data from 400 days of pilot testing and a comprehensive review of international practices. This research and development part of the project played an important role in eventually determining the process train and establishing important design parameters. In this regard, research was focused on the optimal removal of organics and harmful pathogens from the water. This was achieved by operating an ozone/activated carbon pilot plant, a membrane pilot plant and performing some full-scale studies on the old Plant with regard to the precipitation of organics at various pH levels.
Following the research part of the project, water quality concerns were categorized into the following

• Physical and Organoleptic

• Macro Elements

• Microbiological

• Disinfection By-products


Design

The process design was one of a tmultiple barriers process where individual barriers were established for each of the groups mentioned above. With a major emphasis being placed on a safe final product, three barriers were set for pathogens like Cryptosporidium and Giardia. The reduction in organic content before chlorination to reduce the formation of trihalomethanes (potentially caranogenic) also necessitated three barriers. Employing this approach for each concern would ensure a final water complying with the stringent standards required.
Following the establishment of a design philosophy, the processes required to create the multiple barriers had to be identified and sequenced into a final process design. The old Plant was taken as a starting point for processes currently employed and a detailed evaluation of the existing Plant indicated that the old treatment train comfortably provided one treatment barrier against physical and organoleptic parameters through Dissolved Air Flotation (DAF), settling, and filtration and two barriers against most microbiological and biological parameters (two chlorination steps).
To enhance the multiple barrier concept further, the following specific adaptations/additions to the original process were implemented

• Powdered Activated Carbon (PAC) dosing, should one of the key processes such as ozone, membrane or Granular Activated Carbon (GAC) filtration fail. This process will only be an operational barrier.

• The settling step was shown to add very little benefit to the phase separation already achieved by DAF and could be taken out without any compromise to the overall plant performance.

• Ozone was as an additional step before GAC for its ability to destroy cysts, particularly Giardia and Ct-yptosporidium; further to oxidise organics, iron and manganese.

• Biological Activated Carbon (BAC) to enhance dissolved organic removal without the regular high cost of regeneration of GAC.

• An additional barrier was required for both of the biological parameters (protozoan cysts) and the physical parameters (turbidity). This was achieved by membrane filtration.

• The removal of iron and manganese was specifically addressed, especially in view of its fouling effect on the membrane process.

The above philosophy resulted in the process train as shown in the diagram.

To view pictures on the Water Reclamation plant, please CLICK HERE