2.7 What is the space requirement, and how can it be determined?

For a basic assessment of space requirements, approximate design values exist. These values are for domestic wastewater, after pre-treatment, considering different climate conditions.

Parameter	Horizontal Planted Gravel Filter	Vertical Planted Gravel Filter
Warm Climate (annual average >20°C)	3 m2/PE(1  /  20m2/m3	1.2 m2/PE(1  /  10m2/m3
Cold Climate (annual average <10°C)	8 m2/PE(1  /  35m2/m3	4 m2/PE(1  /  20m2/m3

If the constructed wetland is smaller than the recommended value, an overload situation can occur, resulting in operational problems and reduced treatment performance. On the other hand, oversized constructed wetlands are more robust and have a more reliable treatment efficiency but are unnecessarily large and expensive.

The values indicate that the space requirement for horizontal filter systems is generally higher than for vertical systems. Firstly, this is due to the minimal external oxygen transfer in horizontal systems. Oxygen supply is important for the efficiency of the treatment, and a larger area achieves improved contact with air for HPGF. Secondly, horizontal systems have a smaller inlet area. For HPGF, the inlet area is the width of the filter bed multiplied by the bed depth. In comparison, a VPGF uses the entire surface as an inlet area.

For the specific design and determination of space requirement, the following formula (by Kickuth) can be applied:

Where,

Parameter	Description	Unit
A	Surface area of filter bed	m2
Qd	Average daily flow rate of wastewater/sewage	m3/d
Ci	Influent BOD concentration	mg/L
Ce	Effluent BOD concentration	mg/L
KBOD	Rate constant	m/d

K_BOD is a rate constant that can be determined using the following formula:

Where,

Parameter	Description	Unit
KT	K20 x (1.06)(T-20)	d-1
K20	Rate constant at 20°C	d-1
T	Operational temperature of the system	°C
d	Depth of water column	m
n	Porosity of substrate medium	%

K_BOD is temperature dependent, and the BOD degradation rate generally increases by about 10% per °C. Thus, the reaction rate constant for BOD degradation is expected to be higher in summer than winter or in warmer climates than colder climates. In consideration of different depths and porosity of substrate medium, K_BOD gives different results for HPGF and VPGF.

Let us practice and apply the formulas to determine the required area of a planted gravel filter!

Exercise to calculate the space requirement of a Planted Gravel Filter

A small community decided to construct a vertical planted gravel filter to treat 10 m³/d wastewater coming from their septic tank. The influent pollutant concentration is 150 mg/L, and the effluent pollutant concentration should be reduced to 10 mg/L.

The community is located in Bangladesh with an average annual temperature of 26°C. It was decided to build a filter system with 80 cm depth. The porosity of the substrate medium that will be used was assumed as 30%.

Using the Kickuth formula, the planner intends to determine the required space for the filter system.

Calculation:

Firstly, determine the rate constant, K_BOD :

Where,

Parameter	Description	Value in Exercise	Unit
KT	K20 x (1.06)(T-20)	1.56	d-1
K20	Rate constant at 20°C	1.1	d-1
T	Operational temperature of the system	26	°C
d	Depth of water column	0.8	m
n	Porosity of substrate medium	30	%

The rate constant K_T was determined on the assumption for the BOD degradation rate K₂₀ to be 1.1 per day.

Secondly, determine the required surface area.

Where,

Parameter	Description	Value in Exercise	Unit
Qd	Average daily flow rate of wastewater/sewage	10	m3/d
Ci	Influent BOD concentration	150	mg/L
Ce	Effluent BOD concentration	10	mg/L
KBOD	Rate constant	0.37	m/d

Answer:

The vertical planted gravel filter will require under the given conditions a space of approximately 74 m².

Determining the space requirement further needs to consider that the filter system should be divided into several beds in consideration of the recommended organic and hydraulic load. The beds can be used in an alternating manner.

While vertical systems are equipped with a piping system that distributes the influent on the entire surface area, a horizontal filter only receives it at the width of the entering side of the bed. Considering this design aspect, the following recommendations can be considered when designing an HPGF:

• A bed is recommended to not exceed a size of 100 to 200 m² for space design.
• The maximum length is recommended to be 10 m, with tolerances up to 20 m for wastewater with high organic concentrations.
• The maximum width is recommended to be 10 m to respect a good distribution (plug flow).
• The bottom slope of 0.4% (up to 1%) corresponds to the hydraulic gradient.

If a higher treatment capacity is required, installing several filters in parallel is advisable.