Case Study: Žminj
ˇminj is a municipality in the central part of Istria (Croatia), 15 km south of Pazin, population 3.447. In 2002 a constructed wetland was built to treat the wastewater of 750 p.e.. The layout of the system is as follows: pretreatment in septic tank, a 3-stage constructed wetland: horizontal subsurface flow, vertical subsurface flow, horizontal subsurface flow. The total surface area of the constructed wetland is 1700 m². The system was planted with Phragmites australis and Juncus effusus.
Today the system is clogged, causing overland flow throughout the complete system. This results in a lot of nuisance.
The main reasons for the ocurring problems are: insufficient capacity of the septic tank, improper design of the septic tank, improper design of the distribution systems in the different constructed wetlands.
Some changes were proposed to improve the functioning of the constructed wetland, within the limits of the budget of the project. First of all changes had to be made to the septic tank to optimize the residence time and to avoid the discharge of the scum layer in the constructed wetland.
As for the constructed wetland two proposals were developed. Both based on a layout consisting of a horizontal subsurface flow CW, a second horizontal subsurface flow CW, finishing with a free water surface wetland planted with Lemna minor.