E1592 V 6 Non Technical Summary
Project Overview
Public Disclosure Authorized The town of Ogulin in northern Croatia has long-held problems of wastewater and stormwater treatment and disposal. At present the majority of the households discharge their wastewater without prior treatment into septic tanks or directly into the nearby sinkholes where it enters the local groundwaters. Stormwater drainage is disposed of in a similar manner, and the increase in hard surfaces that has accompanied urbanisation has resulted in not only a considerable increase in the flow of stormwaters, but also in their levels of pollution. Heavy precipitation also commonly results in clogging of sinkholes and local flooding, which in turn impacts on local hygiene, ecology and economy.
The proposed project would involve the establishment of a 13 km trunk sewer and associated branch lines for the collection of domestic and industrial sewage effluent from the town of Ogulin and stormwater from high flood risk area. Effluent from the
Public Disclosure Authorized proposed sewer infrastructure will flow to a new tertiary wastewater treatment plant which will use biological means to treat the sewerage effluent to an acceptable standard before discharge to the groundwater environment via an adjacent sinkhole1. The plant will produce a stabilised sludge, which will be disposed of to an appropriately engineered domestic landfill.
The proposed sewer system and wastewater treatment plant will be developed in a number of stages:
Phase 1: An initial trunk sewer has already been installed to collect stormwater from high risk flood areas of the town. The stormwater effluent will be pumped in phase 2 to the wastewater treatment plant, where it will be treated mechanically via an oil and sand trap before discharge to the adjacent Public Disclosure Authorized sinkhole.
Phase 2: Under this phase further sewers will be built, together with the wastewater treatment plant to allow the treatment of wastewaters to a tertiary standard. The plant will use an activated sludge and membrane filtration methodology before discharge of treated effluent to the adjacent sinkhole. The sewers constructed in phase 1 will be extended in phase 2 to become a partly combined sewerage system.
Subsequent to the commissioning of the phase 2 tertiary treatment plant it is envisaged that further domestic houses and industrial premises will continue to connect to the sewer infrastructure thereby decreasing the regions current reliance on septic tank infrastructure. It is envisaged that during the transition phase contents of septic tanks will be able to be treated in the plant. Public Disclosure Authorized
1 Such solution has been confirmed by exploring the underground and testing ground absorption in abysses, presented in the study “Geomehani ko-geofizi ki istra ni radovi - ure aj za pro iš avanje otpadnih voda grada Ogulina“, prepared by “Geokon“, Zagreb (October 1999, no. E-090-99), as well as by previously prepared studies and tests conducted on the field.
Project Setting
The town of Ogulin is located in the Ogulin-Plaški valley, a hilly area some 300 m above sea level, and rising to 500-600 m in higher elevations. The valley is comprised of two karst fields, the Ogulin and Plaški Fields, made of highly pervious layers of limestone and dolomite, characterized by long cave systems such as the ula’s Precipice- Medvednica Cave complex.
The Ogulin administrative region covers an area of 542 km2, comprising about 15% of the total area of the County of Karlovac. The town itself belongs to a sparsely populated area of Croatia and according to the 1991 Population Census had a population of some 16,732 people (9.1% of the county’s population). The town’s economy is based primarily on transportation, and, traditionally on forestry and agriculture, although tourism is becoming increasingly important, with the Croatian Olympic Centre Bjelolasica and Lake Sabljaci located nearby.
Ogulin lies within the catchment of the Dobra River, which flows into the town from the west in a deep gorge, before sinking underground in a large sinkhole, known locally as the ula ponor. Once underground, the river flows through an extensive cave system before emerging approximately 4km to the north-east and flowing in a general north-easterly direction towards Karlovac, approximately 40km distant. The area is considered highly sensitive and the potentially grave consequences of local pollution mean that the construction of the sewerage system and wastewater treatment works have been long been a priority of the town. In addition, and approximately once per year after intense rainfall events and Spring snow melt, the capacity of the Dobra sinkhole is insufficient to cope with the high river flows generated, resulting in flooding of the town centre.
With the exception of the drainage features associated with the Dobra River, the surface drainage network tends to be poorly developed due to the permeability of the surface soils and underlying bedrock. Less intense rainfall tends to infiltrate the ground rapidly, where it causes dissolution of the carbonate substrata and results in the formation of dissolution features and the typical karst topography. As a result, most of the drainage infrastructure within the town, such as from rooftops and highways, has historically been to soakaway, and emerges as springs in the valley of the Dobra River, although some is also known to emerge in the valley of the Mre nica River, to the south-east.The current discharge of wastewater and sewage to solution features, is reflected in the water quality of the main springs around the town, which show evidence of high concentrations of faecal bacteria2.
Croatia is amongst one of the most biologically rich countries in Europe and supports an unusually high concentration of endemic species, with the karst ecosystems considered especially valuable. The aquatic habitats within the system support a unique flora and fauna with several endemic species of fish, invertebrates and amphibians of particular note (for example some 11 species of fish are known to live
2 Data from five of the main springs located in the vicinity of the town (Tounj