Pollutant Removal by Horizontal Subsurface Flow Constructed Wetlands in Lithuania

Abstract

Horizontal subsurface flow constructed wetlands recently have been developed in Lithuania. They ensure the efficient removal of nutrients and organic matter. This article analyses BOD₅ and N and P removal efficiency with respect to filter loads. To achieve a normative wastewater treatment level according to BOD₅ (25 mg O₂ l⁻¹), loads of filters should not exceed 5.8 g m⁻² day⁻¹, that is, the treatment of wastewater produced by population equivalent requires a filter area of 10.5 m². After treatment in filters total N of wastewater contains 55–85% of mineral N. Nitrite-N and nitrate-N make up 0.1% and 3.0% on average; the other part is composed of ammonia-N. Increasing total N concentrations contained in water outflow predetermine higher ammonia-N amounts. If water outflow contains 10 mg l⁻¹ of total N, ammonia-N makes up 51.5%; if it contains 40 mg l⁻¹, then ammonia-N makes up 81.0%. The average total nitrogen removal efficiency is 37–44%. P removal is affected by the physico-chemical characteristics of sand and qualitative composition of phosphorus contained in wastewater. P removal up to 2 mg l⁻¹ from domestic and other wastewater of similar composition requires no higher than 0.15 g m⁻²/day⁻¹ load of filters according to total P.

Key Words:

• Wastewater treatment
• Constructed wetlands
• Reed bed filters
• Horizontal flow.