Aerated Engineered Wetland (AEW) for domestic wastewater treatment

Aerated Engineered Wetlands (AEW) typically consist of the following main components also illustrated in Fig. 1 below:

• Plastic liner
• Gravel layer, coarser on the bottom of the bed with diameter of 8 - 15 millimetres (mm), and 15 - 20 centimetre (cm) of mulch layer (only for cold climates)
• Vegetation
• Drainage system
• Distribution system
• Aeration system (air blower and distribution grid)

Fig. 1: Schematic illustration of an AEW. Source: Own illustration (2020)

Aerated wetlands are generally designed with a medium-fine gravel media with a height of 100 - 200 cm; the water level is regulated by the effluent standpipe and kept 5 - 10 cm below the surface of the gravel layer (DOTRO et al. 2017). Wastewater enters the AEW after preliminary and primary treatment (see for example Settler or Anaerobic Systems for Domestic Wastewater Treatment), the flow in these beds may be horizontal or vertical. The coarse bubble aeration network is placed under the gravel substrate of a Sub-surface Flow Wetland basin, and a blower supplies air to it. This eco-technology allows the removal rates of biologically-oxidable contaminants (e.g., Ammonia, BOD) to increase to almost complete elimination levels (HIGGINS et al. 2011).

AEWs generally have an up to 5 times smaller surface area than equivalent passive sub-surface wetlands. AEWs are capable of achieving >95% removals of most pollutants during summer and winter. The nutrient demands for bacterial growth should always be estimated to provide the appropriate fertilization in case of lack of nutrients. The energy consumption of an AEW depends on the type of wastewater and the associated oxygen demand.

Aerated wetland technology has been available for over twenty years, however, implementation has increased dramatically over the past ten years. Today, AEWs can be found in North America, South America, Europe, Africa, and Asia (NIVALA et al. 2020).

AEWs are characterized by high efficiency removal rates, and by a more flexible design, occupying less space compared to standard CWs. This makes these systems suitable to treat a variety of wastewater types in addition to domestic wastewater, such as industrial wastewater, landfill leachate, airport de-icing runoff, mine tailings and groundwater contaminated with petroleum compounds (DOTRO et al. 2017).

AEWs are efficient even in very cold climates if an appropriate insulating layer of mulch is provided.

AEWs require additional energy input in comparison to conventional constructed wetlands. However, the energy consumption remains at least five times lower than conventional technological solutions (e.g., activated sludge systems). The use of inverters, probes (generally Redox and Oxygen) and PLCs (Programmable Logic Controllers) can allow the regulation of oxygen transfer, optimizing energy consumption.

The costs per surface unit are higher than for Vertical Flow and Horizontal Flow wetlands, but the required surface per person equivalent (PE) is sensitively lower. The costs are around 60 - 120 Euros/PE for large scale systems, going up to 150 Euros/PE for medium size and 200 Euros/PE for small size plants. In developed countries the maintenance cost is in the range of 15 - 22 Euros/PE.

The Operation and Maintenance (O&M) requirements for AEWs are relatively simple and allow management by a community-based organisation or even private individuals – provided someone is skilled to handle blowers and other electromechanical equipment. Most tasks can be performed by low skilled workers as for other types of CWs.

Nevertheless, the system is more complex from a technological point of view, and tasks related to the operation and maintenance of the blowers and other electro-mechanical equipment and associated skill requirements need to be considered when setting up system management.

In case of organic overloading - Organic Loading Rate (OLR) higher than 80 - 100 grams of Biological Oxygen Demand per square metre (grBOD/m²) - AEWs can become clogged.

Fouling of the air distribution lines has been reported in some cases due to iron precipitates or other salts forming at the air distribution orifices, or due to the growing of biofilm inside the emitters. Therefore, the monitoring of the air pressure is important in order to intervene with proper cleaning procedures when necessary (generally using chloric acid, bleach, hydrogen peroxide or other chemical product selected on the basis of a deepen analysis of the fouling causes). The replacement and repair of the air distribution system can be complicated as the distribution lines are buried under the filling material (WALLACE et al. 2020).

Maintenance includes periodic control and emptying of sludge and scum in the primary treatment system, plant harvesting (usable as biomass for energy production, or as building material for thermal and acoustic insulation), checking of the perfect functioning of the distribution system and of the aeration system (pressure monitoring), regulation of the air flow according to inlet wastewater characteristics, ensuring that no clogging occurs in the bed, and sampling of the discharged water.

To the knowledge of the authors, no significant experiences with this technology in India are available.

As part of the EU-India cooperation project NaWaTech (see NaWaKit for Natural Water Systems and Treatment Technologies to cope with Water Shortages in Urbanised Areas in India) a small AEW was constructed in the Dayanand Park, Nagpur . The CW treatment system consisting of four parallel lines - one being an AEW - was implemented in 2015 - 2016, treating around 25 m³/d. The total capacity of all treatment lines was about 100 cubic metre per day (m₃/d) corresponding to about 1,000 PE. The system was designed to meet the discharge standards for inland surface water imposed by the Central Pollution Control Board. Capital costs for the whole pilot system were: Euros 107.000 (US$ 126.000), O&M costs were around Euros 16.500 (US$ 20.000) (GARFÌ 2015).

NIVALA et al. (2020) state that “Input from the international treatment wetland community reveals that there are currently nearly 500 aerated wetland systems operating worldwide. The countries with the most aerated wetlands are the USA, followed by Denmark and the United Kingdom (UK). Application of aerated treatment wetlands in European countries is steadily increasing (mainly for treatment of domestic wastewater), and first pilot-scale or full-scale systems have been constructed in Africa, Asia, and South America”. The number of AEW systems and type of wastewater treated worldwide is illustrated in Fig. 2 below:

Fig. 2: Number of AEW systems worldwide and type of wastewater treated. Source: NIVALA et al. (2020)

In a research facility in Langenreichenbach, Germany, six treatment plants treat municipal wastewater, two of which are aerated wetlands, one Horizontal Flow (HF) and one Vertical Flow (VF), with the following key-date (NIVALA et al. 2019):

• primary treatment: septic tank
• VF AEW: 6.2 m² surface area, 85 cm effective depth; HF AEW: 5.6 m² surface area, 100 cm effective depth
• Inflow: 575 L/d VF AEW; 770 L/d HF AEW
• Removal rates VF AEW: 99% CBOD5; 92% TOC; 92% TSS; 56% TN; 98% NH4-N (annual mean parameters, June 2014-May 2015)
• Removal rates HF AEW: 100% CBOD5; 91% TOC; 82% TSS; 48% TN; 100% NH4-N (annual mean parameters, June 2014-May 2015)