Tianjin Culture ParkThis lake will hold and dispense, in slow motion, the incoming rainwater which cannot be drained away. The high dust content in the rainfall means the organic input into the water is very high. This increases the lake’s trophic level, and provides phytoplankton with excellent conditions for growing. This would result in rapid growth of algae, and deteriorate the appearance of the central system, which, in the worst case scenario, could lead to the water overflowing and creating a strong smell. For this reason, a water plan guaranteeing good water quality has been devised. On the one hand, water will continually be drawn from the lake and treated, and on the other, the incoming rainwater will be treated before entering the lake. This will ensure that as few nutrients as possible find their way into the circuit. The phytoplankton heavily depends on these nutrients to grow and multiply; phosphor particularly plays the limiting role in this case. A phosphor concentration below 0.03 mg P/l is thus aimed for. However, the rain already contains 0.3 mg/l, which highlights the importance of treating it before it enters the lake.
The rainfall from adjacent areas, which cannot be directly drained away, is collected in storage canals around the lake. During the typhoon season, the amount of incoming rain water can rise on the northern wetland to 4320 m³/h and on the southern wetland to 720 m³/h. Apart from the rainwater collecting storage channels, four skimmer are also connected to the northern and the southern wetland. There are two skimmers drawing water on the North-East corner and two on the South-East corner. In total they are able to hold up to 400m³/h, 200m³/h are transported to the northern wetland and also 200m³/h to the southern wetland with the function to keep the lake water in a permanent water treatment circuit.
The pump shaft communicationThe main elements of the water treatment circuit are shafts with pumps, where water can be transported. In this case the rain water as well as the skimmer water is mainly transported to northern or the southern wetland on the western lake border. Should a typhoon cause a greater rainfall than the amount of water which is normally transported away by the pumps, the water level inside the shafts increases. This is realized by a pressure sensor, which has to be installed in every shaft. As soon as the pressure sensor records an inflow of rainwater, the pumps are powered up in proportion to the amount of the water level. Rain water amounts until the three year rain event can be pumped on the wetlands and passes the soil filter till the drainage which leads to the lake. In case of higher rain events, the storage canals are equipped with emergency overflows, also in lake direction.
The skimmer systemThe skimmers are fitted with a variable overflow lip. They thus draw-off the surface water to remove the dirt, rubbish and leaves floating on the surface. The inflow of water from the skimmers can be individually restricted by the pumps behind the skimmers, which are particularly also started up to extract the surface dirt. The water from the skimmer is collected in the skimmer shafts where the dirt is separated by a stainless steel tube. The skimmer shafts have to be cleaned once in a while by a high pressure suction vehicle. After passing the stainless steel tube the water is pumped from the skimmer shafts threw the skimmer pipes: one on the northern lake border and one on the southern side, to the respective constructed wetlands. Two skimmers are connected at a time by one pump, which is able to transport 200 m³/h. So there are two pumps for the four skimmers, one on the northern side and one on the southern lake border.
within the water treatment system there are two wetlands with 1911 m² (north) and 1961 m² (south). On the surface of the constructed wetland, the water is distributed over the filter material through several outlets, 73 water distribution plates on each filter. Should the permeability of the upper filter layer be too drastically reduced by the input, and should the water dam up in the event of high volume flow rates, the water can drain off directly into a wetland overflow shaft. However, this once again results in partially unclean water entering the system. The water normally trickles vertically through the body of the filter, where it is cleaned as part of physical and biochemical processes. Drainage pipes are laid in the base of the filter to collect the water and lead it to the lake which is located lower down.
The constructed wetlands
The Mixing nozzle systemThere is also a system of five mixing nozzles in the middle of the lake. They have the function of a venturi injector and serve to mix fluids and solids. In this case they serve to introduce the precipitations agents to the lake water.
The precipitation agents Lime and FeCl are added to the skimmer water in shaft 2.1. Then the mixture is transported towards the mixing nozzle pipe beneath the lake surface to these nozzles with an inflow controlled by pump 2.3. The mixing nozzles are capable to introduce a volume flow around 20m³/h each and to mix the introduced fluid with a three times higher amount of lake water at the entering point. Like this, the entering precipitation agents are distributed within the lake water in a high efficient way.
The Precipitating agentsPart of the biomass will also be precipitated directly in the lake, and then permanently bonded in the sediment. For this reason, lime water and iron chloride or rather the biological flocculant Heppix are added as precipitating agents. Combining both precipitating agents will prevent the pH value from changing too drastically. The substances are added via the mixing nozzle system (chapter 1.4), and particularly apply if there is evidence of rapid algae growth, in order to bind as much biomass as possible.
Metering:Precipitation agent/ Lake water: around 20 mg/m³ or
Lake water transport to mixing nozzles: 100 m³/h è dosage precipitation agent 1-10 g/h
1.6The Deep drain system (deep water release)In the case that the lake water level increases above 2.5m (asl), a deep drain pipe gives the possibility to draw deep lake water. There are installed two perforated pipes (75m each) on the deep area lake ground on -0.80 m (asl). They are connected to two pump shafts, one on the north eastern side and one on the south east side on around 0.0m (asl). From these shafts the water is pumped to the sewage canal.