The goal of this project has been to analyse the
influence of future sewer system overflow on the river Sava, from
the point of view of total pollutant load and possible extremes.
In 1998 a procedure about building Zagreb central wastewater treatment
plant has been initiated. Parallel with this project, Zagreb Sewer
System Optimisation Project Phase II has been completed, and it
represents the main documentation for long term development of
the city sewer system. The present sewer system collects and passes
waist water until the outlet to Sava without a previous treatment.
Transport of water is performed by collecting all kind of wastewater
together. There are no weirs or controllable structures for run-off/inflow.
For calibration/verification needs and also for simulating various
scenarios the following measures were used: rainfall data from
four rain gauges, level and discharge at the sewer system from
three locations, data on industrial waste water quality and quantity,
sampling and water quality analysis at three locations at MDC
(GOK), Sljeme's creeks water quality, discharge and water quality
at three locations at river Sava. The River Sava and Zagreb sewer
system water quality mathematical models were created and later
on calibrated and verified from hydrology-hydraulic and water
quality aspect. Simulations of pollutant variable in time and
space were done for catchment area and sewer pipe. They were used
as boundary input data for pollutant simulations in the river
Sava model. Different loading combinations were considered (typical,
critical,…) for present and future city development. Analysis
of pollutant variable in time and space was done for the sewer
system for characteristical working conditions and pollutant quantities
(total and extreme) that through weirs and outlet go to river
Sava. Through this analyses the following conclusions were made:
- By building and activating sewer system weirs water quality
of the river Sava will not be changed. - By anticipated city development
total pollution BOD will increase for 16% comparing to the present
situation. - By building wastewater treatment plant BOD will be
reduced for 90% (assumption), so the total pollution BOD from
sewer system to the river Sava will be reduced for 88% (for critical
load combination).
