The primary need in the town was to replace an old wastewater treatment plant that had expensive operations and inadequate treatment capacity. Furthermore, the original treatment plant was situated on top of a nearby hill, requiring all wastewater from the town to be pumped there. The illogical location of the wastewater treatment plant would have meant pumping approximately 3,600 m3 of wastewater daily.

Under a World Bank program and following a feasibility study, the Moldovan government decided to replace the original activated sludge plant with a constructed wetland system with a total capacity of approximately 26,000 PE (with plans to expand the plant to up to 33,000 connected inhabitants by 2022) and several food processing plants. During the peak season of the food industry, the constructed wetland system will be able to handle a maximum organic load of 2,000 kg BOD5 / day. Initially, the average load of the treatment system was 700 kg BOD5 / day (11,600 PE – population equivalent) with a maximum plant load of 1,200 kg BOD5 / day (20,000 PE).

The primary sedimentation phase (e.g., a septic tank as is customary for smaller wastewater producers) was skipped in the case of the Orhei plant because it takes place directly in the secondary processing stage using the so-called French system. The system consists of filter beds connected in 4 lines. These beds operate independently of each other and are always arranged in pairs.

Stage 1 treatment – the French system, which is essentially a variation of a vertical flow filter, but raw wastewater is applied to it without prior pre-treatment. An aerobic sludge layer forms on the surface, overgrown by plant roots, which must be removed every 10-12 years.

Stage 2 – a vertical subsurface flow filter that completes the biological treatment process. The total area utilized for the treatment process is approximately 5 ha. It is certainly one of the largest constructed wetland wastewater treatment plants for municipal wastewater in the entire world!

Due to local conditions, it is necessary to pump wastewater within the treatment plant and subsequently pump the treated water over a distance of 1.5 km. Pumping costs reach a value of 0.4 kWh/m3 of wastewater, which is not so bad. For comparison, activated sludge plants range in electricity consumption somewhere between 0.8 and 1.4 kWh/m3.

You can read about the largest wastewater treatment plant in the world on our blog here:

Developing countries often need a system with simple technology for wastewater treatment in order to reduce operating costs as much as possible and achieve high efficiency without requiring highly qualified and specialized staff. The proposed system is characterized by low energy consumption and very low waste material production. Sludge that accumulates in the plant over the years is used as a soil conditioner (fertilizer). This means that organic material stabilizes over the years and, once applied to the soil, returns to where it originally came from.