Experiences with hybrid constructed wetland and pond system for polytypic wastewater treatment

CWs are increasingly being employed for treatment of wastewater, sludge and industrial effluent as a cost-effective, low energy and robust alternative to traditional engineered biological treatment, such as the activated sludge process. CWs are classified according to their mode of operation as surface flow wetlands (SFs), HFs, VFs. They have been often used alone in the treatment of domestic sewage in china with low efficiency. In order to improve the efficiency of wetland treatment, different kinds of wetland such as HF, VF and SF are designed in assembly and labeled as a HCW. HCWs can be used in domestic sewage, industrial wastewater, polluted channels etc. HF provides high removal of organics and suspended solids, but nitrification processes are not as good (Vymazal J., 2005). However, VF does provide a good condition for nitrification, but little denitrification occurs in these systems. HF and VF can be combined as a complementary processes to produce an effluent low in COD, which is fully nitrified and partly denitrified and therefore much lower total-N outflow concentrations (Herrera Meliána et al. 2010; Oovel et al. 2007; Serrano L., 2011). Because phosphorus removal is restricted by the used filter materials (sand and gravel) with limited sorption capacity, an abolishing pond is integrated in some projects for further elimination of phosphorus.

This paper introduces the technological structure of the HCW in the case of tertiary treatment of industrial wastewater in CAMIP and decentralized wastewater treatment projects on Lotus Island in eastern China.

In the VF, a round shape was used in order to generate better scenic effects. The influent enters at the surface through perforated pipes to evenly distribute the inflow throughout the VF surface, the effluent then flows to the bottom and goes to the HF. Through the infiltration process, pollutants are degraded by many microbiological treatment units. The quartz sands are selected and disposed as VF substrates, with depth being about 70–80 cm. Treated water is collected by drainage pipes at the bottom of the VF, and flows to the HF by gravity, using only one time pumping lift in this system.

In the HF, influent enters at the surface and the effluent exits at the bottom. Coarse substrate is disposed at the entrance in order that water could be distributed through the whole substrate and to avoid shortcut flows and clogging.

The effluent of the HF flows into a polishing pond, which includes submerged and floating aquatic plants as well as some kinds of water fauna. This pond system could further purify the effluent from the HF for better clarity, lower P and N contents, and have landscaping effect. The system is becoming a recreational site for local inhabitants.

Sludge from the BT is moved to the sludge dewatering plant bed for drying. The leachate from the sludge bed flows back to the BT and is then treated by the CW system.

In order to achieve water recycling, CAMIP in southern Jiangsu Province built a tertiary treatment system using HCW for the industrial wastewater treatment plant. The treatment process in the treatment plant is ‘hydrolysis + Cyclic Activated Sludge Technology (CAST in short) + activated carbon plant’. And the outflow meets the Discharge Standard of Main Water Pollutants for Municipal Wastewater Treatment Plant & Key Industries of Taihu Area (DB32/1072-2007). The capacity of tertiary treatment is 4,000 m³/d, where the final water quality shall meet level IV of Environment Quality Standards for Surface Water (GB3838-2002).

Table 1 includes a summary of basic data of the four domestic sewage treatment systems.

Table 2 includes a summary of basic data of tertiary treatment of industrial wastewater in CAMIP.

In this HCW system, the VF acts as the as first biological treatment unit, and plays a vital role for pollutant removal. The wastewater is fed by a pump intermittently in order to create a better biological degradation environment with enough oxygen supply. East and West village projects use the same feeding intervals. The other two projects use different feeding intervals, which are based on the wastewater volume and water levels.

All functions of the HCW system are operated by a Programmable Logic Controller (PLC). There is a central PLC at the main control box near the inlet construction. The inlet structure has a weir, a regulation damper and a motor valve for each VF cell. The damper is used for pre-regulating the maximum flow at the weir, whereas the motor gates are used to shut and open the specific flow chambers. The total flow is monitored by an inductive flow meter at the pumping station. All outputs and input of PLC are monitored internally.

Most parameter measurements have been completed during 2010 to 2013. In total, 50 water sampling measurements were performed regarding the four projects. The mean values are summarized and shown in Table 3.

Tertiary treatment of industrial wastewater at CAMIP will be operated in June 2014. The pollutants removal efficiency as designed in Table 4 will need to be tested in the future.

According to Table 4, after treatment of the wetland system, effluent water can reach the level IV of GB-3838-2002.

The characteristics and treatment performance of the HCW with a post treatment pond, of the four projects are studied for the treatment of village based wastewater in the Yangcheng Lake area, Suzhou City. The following conclusions can be drawn:

1. Despite low inlet concentrations due to high amounts of ballast water, removals obtained by the HCW systems were very high for COD (88.6%, 74.2%, 84.0% and 90.2% respectively), (96.7%, 95.8%, 98.7% and 87.2% respectively), SS (81.8%, 90.5%, 91.5% and 52.6% respectively), TN (64.6%, 74.2%, 76.9% and 75.2% respectively) and TP (77.7%, 85.6%, 88.2% and 73.1% respectively). The most interesting results of this study are the eliminations of ⁠, TN and TP. Removal rate of could achieve in some cases more than 90%. The high nitrifying ability of the VF is the main cause of elimination. TN and TP could achieve in some cases more than 75% and 88% removal respectively. The multi-stage treatment units contributed to the reduction of TN and TP.

2. Although the experimental HRT of the VF were much lower than those of the HF, the VFs were in general more efficient, particularly for and COD. The contribution of the HF to water purification was important in the elimination of TN, SS and turbidity. However, these results are valid only for the configuration used in this study and cannot be generalized.

3. Planting different kinds of aquatic plants introduced few significant differences for treatment efficiencies. Maybe a further study for this should be done in the future.

Industrial sewage, instead of domestic, has complex composition. Based on the outfall water of an industrial sewage treatment plant, using a natural ecological treatment system according to local conditions can maximize the use of sewage. It is feasible to adopt CW technology, which is simple, without professional management, easily maintained, cheaply operated and has many environmental and social benefits, for the treatment of outfall water from industrial sewage treatment plant. This project makes full use of the special cleaning feature of each unit of VF, EP, SF, SAF in order to achieve maximum treatment capacity. The implementation of this project can reduce water pollution risks to the Yangtze River, and protect the safety of drinking water sources downstream.

During the actual operation of the project, related experience on construction and management will be summarized in time to form a complete management practice of ecological sewage treatment systems. This treatment wetland has the capability to demonstrate best available technology under real conditions in large technical scale. Scientists and interested public as well as government officers are welcome to visit the onsite training and research facilities including laboratory, online monitoring station and the wetland centre with a viewing platform on top of the building.

The authors would like to thank the project which financed the work. The project name is ‘study on the demo project of tertiary treatment of industrial wastewater treatment plant using constructed wetland technology’, and grant number is ‘BE2013645’.