REED BEDS - AN INTRODUCTION

SCALE                                                                                                                                                           Constructed reed beds can be created for most scales of operation, from the very large to the very small with the size determined by the wastewater to be treated, the volume, the organic / pollution loading of the liquid and the location.

HOW ARE THEY MADE                                                                                                                                               In general reed beds are constructed out of natural local resources, such as earth banks and clay liners or they can be “more constructed” using some form of containment wall, such a concrete blocks or timber frame with an impermeable rubber/butyl liner. The void space is generally filled with graded gravel or soil gravel matrix and planted with Phragmites australis or other hydrophytes at close spacing. These plants encourage opening up of void spaces to allow waste water to progress through the matrix, as well as facilitating oxygen transfer to aerobic zones in the matrix and providing a sink for nutrients.

TYPES OF REED BED                                                                                                                                                   There are several types of constructed reed bed, but the two main types being horizontal and vertical flow, each performing slightly different tasks. Ideally reed bed systems should be constructed to make the most use of any fall to allow the movement of water through the system via gravity, this reduces operating costs and the reliance on power for pumps and flow systems.

WHAT DOES IT TREAT                                                                                                                                              In essence constructed reed beds treat liquid waste by removing organic matter, reducing BOD:COD, oxidising ammonia, reducing nitrate and removing phosphorus. The processes are complex involving bacterial oxidation, filtration, sedimentation and chemical precipitation. 

HOW DOES IT WORK                                                                                                                                      The hydrophytes grow and develop a large root zone, which provides an extensive habitat for microbial activity. As the wastewater passes through the reed bed it comes into contact with a system of aerobic, anaerobic and anoxic zones. Around the plant roots is an aerobic zone generated by the plant passing oxygen down through its roots and rhizomes where large populations of aerobic bacteria effect the biodegradation of the organic matter in the effluent.

The rhizomes and roots grow vertically and horizontally opening up the bed to provide a passage for wastewaters. Soil and gravel provides a growing media for the plants as well as providing a medium to which complex ions attach, aiding their removal from the waste. This will remove heavy metals and additional contaminants form the waste stream.

Soil has a diverse microbial fauna. Research work has shown that microbial species that find reed bed conditions attractive, (microbes such as bacteria, fungi, algae and protozoa), are capable of naturally biodegrading synthetic chemicals such as pesticides, chlorinated hydrocarbons, and even phenol, methanol, acetone and amines from industrial effluents.

These microbes can also act as predators to pathogenic organisms that can cause disease in animals and humans, as well as utilising and altering nutrient sources present. Suspended solids from the waste stream collect at the surface and are aerobically composted in the above ground debris of the bed.

Constructed reed bed

Floating reed bed