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The horizontal flow (HF) reed bed is the type that has been most commonly used to date. They are described as horizontal as the effluent enters one side of the bed and moves in a continuous process across the bed where treatment is effected by micro-organisms before being collected and discharged on the far side of the bed.

HF systems primary removal mechanisms are filtration, sedimentation and microbial activity, ideal for the removal of suspended solids, pathogens and BOD. The wastewater passes through aerobic, anoxic and anaerobic zones.

The aerobic zones are situated close to the roots and rhizomes of the reeds. Oxygen is transported to this zone from the shoots and leaves of the reeds. The wastewater passes through the media (gravel, sand, soil) where there is little oxygen availability creating anoxic and anaerobic zones. With the absence of oxygen in these zones, anaerobic organisms breakdown organic molecules using metabolic processes that are not oxygen dependant. 

Both the process and physical design of reed beds has changed significantly over the past few years to increase the cost-effectiveness of these systems. These developments have been assisted by work completed by ARM Ltd.

There are two different types of horizontal system. The sub surface flow and the surface flow systems. By far the most common type is the sub-surface flow which is often used for final polishing or tertiary treatment applications. Surface flow reed beds are often used for metal removal and settlement applications.