Trickling filters as an ideal sewage water treatment solution

Posted on 13 Jun 2016

Trickling filters as an ideal sewage water treatment solution when re-evaluating the suitability of reed beds

The sustainability and operation of reed beds or ‘planted filters’ is often subject to re-evaluation. This re-evaluation relates mostly to Northern Europe and other regions which experience colder winters.

Many systems of this type freeze during especially cold Northern European winters. This results in a  pollution hazard for the owners of these systems.

Trickling filters such as the BIOROCK system are now becoming generally accepted as the ideal non-electric sewage treatment solution by wastewater professionals who have re-evaluated the suitability of reed beds.

To fully understand the advantages of a “vertical” trickling filter when compared with a reed bed it’s important to go back to the basics of the technology:

Reed beds have been developed for the past 25 years as a green and natural solution for treating sewage effluent. These systems reduce the level of BOD (Biochemical Oxygen Demand) and SS (Suspended Solids) in sewage to very low levels.

Reed bed design

There are several different reed-bed designs (Horizontal flow only, vertical flow only, or multi-stage systems incorporating several stages of horizontal or vertical flow) These can be used for septic tank effluent treatment or for tertiary treatment after a sewage treatment plant.  Typical reed bed applications would include treating the discharge from a septic tank or a treatment plant (acting as a final polishing filter). The design depends on several factors such as the strength of the incoming effluent and the type of treatment required (ammonia, BOD and SS levels)

Reed beds - operating principle

The common reed used in a planted sewage filter bed develops micro-organisms that digest the pollutants in sewage. This bacteria’s development is achieved by transferring oxygen from the plant leaves down through its stem and roots resulting in bacteria growing in the gravel bed. The sewage effluent flows through this gravel bed and the micro-organisms treat the pollutants. Reed bed systems can be 100% non-electric if the site has a reasonable gradient; however the use of electric pumps can be necessary to lift up the water in “flat” areas.

Power Requirement

Even if most sites with decent gradients enable gravity sewage water treatment, the need for electrical power can be necessary when designing a reed bed system in order to ensure that the flow of wastewater doesn’t flood the system

Planted filters - life span

Reed beds may be a long term solution when designed and maintained properly; however site experiences show that most planted filters may only be viable for up to 10 years. 

Maintenance of reed beds

As with any sewage treatment plant reed bed systems require regular maintenance to keep them working properly and delivering the expected treatment results.  Reed growth should be controlled and weeds removed on a regular basis to keep the plant healthy, preventing it from rotting and clogging the gravel bed. The primary tank needs to be emptied when required.

Refurbishment of reed beds

Clogging problems normally occur either at the inlet or at the outlet of the bed.  A blockage at the inlet end may lead to the septic tank 'backing-up' to the house. A blockage at the outlet end may cause the reed bed to overflow and the gravel bed flooding.

Refurbishing reed beds is tough work and should include the removal and replacement of both the reeds and the gravel bed. The gravel bed may have clogged up because of heavy wastewater loads and the presence of remaining suspended solids, saturating the bed after some years. The replacement of the reeds and gravel bed is an expensive operation.

Re-evaluating of the suitability of reed beds during winter periods: trickling filters as an alternative to reed beds

In the event of cold winters and especially in cold climate zones, the possibility of reed beds freezing solid is a reality, this has forced companies involved in the design and building of reed beds to re-think the merits of planted filters in areas with a risk of freezing.

Trickling filter sewage treatment plants function as two stage treatment systems. Initially the raw sewage enters a primary tank which provides separation and breakdown of organic solids (Primary Treatment) as would be the case for a reed bed system. The sewage then passes through an effluent filter before discharging into the treatment unit which uses an aerobic digestion process (Secondary Treatment) and filtration process (Tertiary Treatment).

The reed beds acknowledged advantages exist here too (non-electric, green, sustainable and naturally aerated); the main difference is the containment of the filter media in a tank. The filter is hermetically “separated” from the environment. The sewage water is treated “vertically” by the media in the tank instead of “horizontally” by the gravel bed which requires a larger surface area. Numerous benefits result from this method of containing the filter media, such as the reduction in space required and easier effluent sampling when checking the quality of the treated water.

BIOROCK units and Primary Tanks are reinforced as standard making them  structurally sound in high ground water conditions, this is another advantage of the containing the filter media: no risk of the wastewater backing up which is an issue with any blockage at the inlet or outlet of the reed bed.

Returning to the first reason for re-evaluating  the sustainability of reed bed systems:  The risk of reed beds freezing solid in cold climate zones. Trickling filter sewage plants offer a more flexible solution for deeper (away from the risk of freezing) installations.  BIOROCK units can be installed in cold climate areas (such as high mountain areas or those particularly exposed to snow), with certain precautions.

Adapting the depth of the installation

The tanks can be installed at a depth which ensures that the water-dispersion system is out of the ground freezing zone. The use of a BIOROCK tank extension is a perfect solution. This "freezing zone" varies by region, and this essential point should be checked with local authorities.

Insulation of the lids and tank walls for both the Treatment unit and the primary tank

Insulation on the outside of the lid will prevent the water distribution system freezing. The activity of bacteria in the treatment plant generates heat. Once the tanks are insulated from the outside, the heat is kept in the tanks retaining a sufficient temperature to ensure proper operation. An insulating material with a thickness of 0.5 cm minimum should be used.

If the BIOROCK units are installed in parallel they can cater for 50, 75, 100 PE and so on up to 300 PE, offering a real alternative to large-scale reed beds with numerous advantages provided by the trickling filter sewage treatment process.

About costs…

Reed beds are not a cheap or easy option for sewage treatment, the costs involved for a complete non-electric sewage treatment plant are no more than that of reed beds. Substantial savings may be achieved with the installation due to shorter installation, fewer delays and the ease of burying of the tanks in the ground when compared to the often complicated construction of gravel beds and the time-consuming seeding and growing of the reeds.