The experimental investigations were performed by the university of Applied Sciences Ostwestfalen-Lippe (HS OWL) and industrial partners.
The treatment concept comprises two lines: In line 1 the technical treatment units covering a dissolved air flotation (DAF) and an anaerobic reactor (type EGSB) are combined with a vertical flow constructed wetland. In line 2 a two-stage constructed wetland system is put behind the flotation stage. The novel treatment concept has convincingly demonstrated its robustness and its ability to remove organic matter (COD by > 98%), nutrients (Kjeldahl-N by 80-85% and total-P by 50-60%), and cyanide (total-CN by > 99%) from tapioca starch wastewater. The treatment results in line 2 (which is not using the EGSB reactor) where all biological degradation takes place in a two-stage wetland system have been similar to line 1. However, much more space is required for line 2, and no utilization of biogas is possible there.
With respect to the Vietnamese Standard, the requirements on effluent BOD5, total-N and cyanide concentrations can be met. Removal of total-P has been insufficient since the treatment does not include an efficient phosphorous sink. There are two possible solutions: Either a precipitation unit is put behind the EGSB reactor (line 1) or granular material with high binding capacity for phosphorous is used in the wetlands (both lines).
There are several sustainable aspects related to the treatment concept:
1. Removal of suspended solids in the flotation stage and subsequent reuse in agriculture.
2. Reduction of the organic load of the anaerobic reactor which, then, can be designed smaller and operated more reliably.
3. Production of biogas that can be used on-site. The starch drying units need an appreciable amount of heat. When no biogas is available, about 35-40 liter of fuel oil per ton of starch is needed. About 65% of this amount can be replaced by the biogas produced. It would also be possible to burn the biogas in a block-type thermal power station in order to produce both heat and electricity.
4. Utilization of the advantage of wetlands that exhibit high process stability and low operation demands. Their disadvantage is the large area required. However, for small- and middle-sized plants in rural areas it is usually possible to realize this stage.
5. Minimization of material input: Besides adding sodium hydroxide to the waste-water in the EGSB reactor for adjusting pH no chemicals are needed.
It is expected that these benefits will make the process scheme interesting for technical-scale applications in small- and middle-sized plants.
Results from the expert system
The expert system was developed by the University of Braunschweig and Vietnamese partners (IER).
The 280 km long Saigon river, including the Dau Tieng reservoir, and the 34 km long Tay Ninh river are located in the tropical monsoon area. The investigated catchment of the Saigon river upstream of Ho Chi Minh City is more complex and larger than the catchment of the Tay Ninh river. The dynamics of the water and mass balance of both catchments are simulated with the SWAT model. The hydraulics and water quality of the Tay Ninh river are simulated with the 1D-HEC-RAS model. For the Dau Tieng reservoir and the tidal influenced Saigon river, the 2D-CE-QUAL-W2 model is applied. The water quality simulation comprises sediment, nutrients, dissolved oxygen and the biological oxygen demand (BOD5).
Because of the topographic characteristics and the strong anthropogenic influences, the water balance of the Tay Ninh and the Saigon catchment is very complex. The catchments are characterized by a very flat topography with gentle slopes. This character complicates a clear delineation of subcatchments and flow path. Broad flood areas, ponds, irrigation channels and rice fields are responsible for a large retention potential of water in the catchments. In consequence, a large amount of precipitation is exposed to retention effects. In the dry season, the water balance is strongly influenced by irrigation measures, in particular for the cultivation of rice and the low flow augmentation.
Good calibration results could be achieved for all stations for the water level, discharge and the water quality parameters, respectively. A coefficient of determination larger than 0.6 was achieved for all stations. Also the coefficient of efficiency after Nash and Sutcliffe confirms a very good agreement.
Using the above mentioned coupled models, an expert system was developed in the project. The system has a uniform operating system with menu control for input and output processes, and additionally a database system for the control of project data and the storage of simulation results.
The actual state of the catchment and different scenarios were simulated with the expert system. For the numerous other tapioca factories in the catchment, fictitious treatment plants, according to the experimental results from the pilot plant, were installed. Suitable solutions were developed under consideration of ecological criteria.
The expert system will allow responsible Vietnamese authorities and institutions to improve the water quality of the Tay Ninh and Saigon river including the Dau Tieng reservoir. In the future, the system can be extended by the Vietnamese authorities to cover the whole Dong Nai catchment.