2.4 What is the difference between horizontal and vertical planted gravel filters?

When differentiating according to the flow pattern, the influent can pass through sub-surface planted gravel filters either horizontally or vertically. The different flow pattern relates to different designs and different treatment results.  

In a Horizontal Planted Gravel Filter (HPGF), the influent enters at one end, passes through the filter horizontally and exits at the other end of a large gravel and sand-filled basin planted with wetland vegetation. These filters are often wide and shallow, with plants strategically arranged in rows along the flow path. The plants are usually positioned near the water’s surface, and their root systems extend horizontally into the water. As water flows through these roots, contaminants are removed.

Three zones are to be distinguished: inlet, treatment, and outlet zone. The inlet zone has pebbles with larger diameters (40 – 80 mm) as a filter medium from the top to the bottom of the system to minimise the risk of clogging and to enable the distribution of influent entering through a perforated inlet pipe. The treatment zone has gravel with a smaller diameter (5 – 20 mm) as a filter medium that must be clean without dust or sludge to avoid rapid clogging.

In comparison, in a Vertical Planted Gravel Filter (VPGF), the wastewater enters at the top and flows downward through a vertically oriented bed of planted gravel. Vertical filters are often taller and narrower, with a smaller surface area relative to their depth. In these systems, plants are distributed throughout the entire depth of the filter bed. Their root systems extend vertically along the whole depth of the filter. As wastewater flows from the top to bottom, it encounters roots, where contaminants are removed.

Due to the vertical flow, the filter system has improved oxygen distribution compared to HPGF, promoting aerobic conditions throughout the entire filter depth. Nonetheless, there is a higher risk of clogging as wastewater must pass several treatment layers: filtration, transition, and drainage. For effective treatment and to minimise the risk of clogging, it is important to respect substrate properties, effective grain size and the uniformity coefficient when selecting filter media. Using sand for the filtration layer shows an increased permeability reduction due to sediment accumulation, but sludge accumulates at the surface, avoiding a higher depth of clogging.

The two filter systems have different operation modes to achieve an efficient treatment. The horizontal filter system requires feeding in small, consistent doses of effluent throughout the day (continuous feeding). It is crucial to prevent the sudden discharge of large amounts of wastewater at any cost. On the other hand, the vertical filter system requires a cycle of alternating between feeding and aeration/resting periods. Aeration/resting periods should be maintained during this cycle, lasting at least 2 to 3 days, ensuring proper time for biological processes. Some practices even advocate for doubling the time allocated for resting and aeration. The level of aeration is intricately linked to the plant species present, as they actively contribute to sludge mineralisation. Moreover, the plants enhance aeration by moving the stems, which are swayed by the wind, promoting increased infiltration. To ensure sufficient aeration time between batches, the VPGF is usually divided into cells by separation with walls or membranes, except for very small single-family systems.

Both HPGF and VPGF contribute to cleaner effluent. Still, the choice between these two filter types depends on factors like space availability, design objectives and the specific requirements of the treatment process.

The following table summarises the differences between the two types of planted gravel filters.

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