The effects of pH, temperature, dissolved oxygen (DO), and flow rate on the phosphorus (P) release processes at the sediment and water interface in rainwater pipes were investigated. The sampling was conducted in a residential storm sewer of North Li Shi Road in Xi Cheng District of Beijing on August 3, 2011. The release rate of P increased with the increase of pH from 8 to 10. High temperature is favorable for the release of P. The concentration of total phosphorus (TP) in the overlying water increased as the concentration of DO decreased. With the increase of flow rate from 0.7 m s−1 to 1.1 m s−1, the concentration of TP in the overlying water increased and then tends to be stable. Among all the factors examined in the present study, the flow rate is the primary influence factor on P release. The cumulative amount of P release increased with the process of pipeline runoff in the rainfall events with high intensities and shorter durations. Feasible measures such as best management practices and low-impact development can be conducted to control the P release on urban sediments by slowing down the flow rate.
A great number of urban centers are drained by a unique sewer network in which wastewater is mixed with urban runoff water in wet weather [
Solids accumulated in sewer systems carry a variety of pollutants. Phosphorus (P), mainly present in sewage as orthophosphate [
Focusing on the latter topic, a number of studies had paid attention to P release from the sediment to various kinds of receiving natural water bodies such as coastal zones [
North Li Shi Road is located in Xi Cheng District of Beijing, where drainage system is consisted of combined sewer system. The sampling site is located in a residential storm sewer of North Li Shi Road. The catchments are densely populated areas with many small retail shops and offices, but little industrial activity. The length of connection pipeline in the sampling site is 500 m with a gradient of 2~3‰. The diameter of connection pipe in the sampling site is 300 mm with 128 mm thickness of sediments. The environment temperature is the highest in July and August ranging from 20°C to 35°C, and 80% of the annual precipitation concentrated in the summer [
The sampling was conducted in dry weather (the fifth consecutive sunny day after the heavy rain) on August 3, 2011. The sediment was taken from the storm sewer at a distance of 0.3–0.5 meters from the inspection well. In the pipeline, sediments with a 3–10 cm width of the cross section were sampled with a shovel. The stones and plastic were removed from the sample. They were then put in air-sealed plastic bags and taken to the laboratory. The samples were kept in 4°C iceboxes for further analysis [
After sampling the sediment, the samples of rainwater were collected directly in the sediment sampling point when the following rain occurred. The rainwater samples were kept in 4°C fridge for simulation of runoff scouring.
The experiment was conducted in 1000 mL beakers with the overlying water at a depth of 6 cm. The samples were run in duplicates. The rainwater samples were filtered to remove the suspended solids and microorganisms [
The water level in these experiments was noted in order to keep the same water quantity after sampling and supplementation. 20 mL samples were extracted with a syringe for the analyses of total phosphorus (TP) in every 10 min. Then, an appropriate amount of rainwater was added to compensate for the loss of water and evaporation. For TP analysis, a water sample was autoclaved at 121°C for 30 min after K2S2O8 was added. Then, 1 mL ascorbic acid and 2 mL molybdate were added and the sample measured using the molybdenum-antimony antispectrophotometric method [
Because an appropriate volume of water sample was collected from the experimental apparatus, and clean water without P was supplied to the experimental apparatus, the cumulative release amount on the
The physicochemical properties were analyzed, including size fraction distribution (Table
Size fraction distribution.
Size fraction (mm) | <0.385 | 0.385–0.076 | 0.076–0.15 | 0.15–0.3 | 0.3–0.701 | 0.701–1.25 | 1.25–2 | >2 |
---|---|---|---|---|---|---|---|---|
Mass fraction (%) | 0.80 | 2.01 | 3.71 | 10.34 | 29.91 | 17.47 | 12.65 | 23.11 |
Various forms of P and their content distributions.
P forms | O-P | IP | OC-P | Ca-P | Fe-P Al-P | TP |
---|---|---|---|---|---|---|
Content (mg kg−1) | 592.4 | 2583.5 | 292.8 | 997.9 | 1198.4 | 3071.0 |
Organophosphorus (O-P); inorganic phosphorus (IP); occluded phosphorus (OC-P); Calcium Phosphorus (Ca-P); iron and aluminum phosphorus (Fe-P Al-P).
Changes in P concentration as a function of pH in the release experiments are shown in Figure
Concentration changes of TP in the pH effect experiments.
The results indicate that the released TP reached the maximum concentration in the overlying water during the first 10 to 20 minutes of the experiment at various pH conditions. Then the concentration of TP began to decrease and finally keep equilibrium. The time to reach equilibrium was about 60 minutes at pH 4 or 6 and 30 minutes under neutral condition (i.e., pH = 7). However, in alkaline medium, the equilibrium time was much longer (i.e., 70 minutes and 80 minutes) at pH 8 and 10, respectively. It was suggested that the equilibrium time of TP concentration was the shortest under neutral condition. The P release from sewer sediments was more stable under neutral condition than other conditions. In neutral conditions, the P in the water can be consumed by some microorganism such as phosphorus-accumulating bacteria through metabolism. However, under the alkaline condition, some metal ions exist at the form of hydroxide gel or inorganic salt in the water. A certain amount of P can be adsorbed by the surface of those forms. In addition, the concentration of TP in the overlying water decreased with slow flocculation and sedimentation. The maximum cumulative amount of P release under different pH is illustrated in Figure
Maximum cumulative amount of P release as a function of pH.
The effect of temperature on P release from the sediments is shown in Figure
Concentration changes of TP in the temperature effect experiments.
However, the concentration of TP in the overlying water decreased gradually and then tended to reach equilibrium. The equilibrium time was about 50 minutes at 15°C and 20°C, and it was about 60 minutes and 80 minutes at 25°C and 30°C, respectively. With the temperature increasing, the activity of microorganisms was increased significantly [
The maximum cumulative amount of P release under different temperature is shown in Figure
Maximum cumulative amount of P release as a function of temperature.
Changes of TP concentrations of the overlying water in the release experiment are shown in Figure
Concentration changes of TP in the DO effect experiments.
This observation can be explained by a certain amount of particles suspended due to the injection of overlying water at experimental preparation phase. The concentration of P was then reduced by the settling and readsorption by the sediment particles. TP concentration gradually increased in anaerobic conditions with the increase of time and then tended to reach equilibrium. The equilibrium time was much longer than that in aerobic conditions. In addition, the maximum release rate of P increased as DO decreased.
Besides time, the Eh of sediments would also be affected by DO. The previous study [
Maximum cumulative amount of P release as a function of DO.
The changes of TP concentration in different flow conditions with time are shown in Figure
Concentration changes of TP in the flow rate effect experiments.
In Figure
Maximum cumulative amount of P release as a function of flow rate.
According to the calculation results above, the maximum P loading is 8.29 mg m−2 min−1, 0.82 mg m−2 min−1, 2.11 mg m−2 min−1, and 178.05 mg m−2 min−1 under different factors (pH, temperature, DO and flow rate), respectively. And, it is clear that flow rate is the primary factor for P release.
The release rules and their affecting factors (i.e., pH, temperature, DO) of P at the sediment and water interface in storm sewer is similar to that in natural water bodies. It was almost not released in the neutral pH condition. However, the release rate of P increased with the increase of pH from 8 to 10 and is much faster at high temperature than lower. The P release was much higher in anoxic condition than that in aerobic condition.
The fitting formulas were developed to describe the relationship between the maximum cumulative amount of P release and the environmental factors in storm sewer. The P release loading in dynamic conditions are much higher than that in static conditions, flow rate is the primary affecting factor. The cumulative amount of P release increased with the process of pipeline runoff in the rainfall events with high intensities and shorter durations. Feasible measures such as best management practices and low impact development can be conducted to control P release on urban sediments by slowing down the flow rate.
The authors declare that there is no conflict of interests.
This study was supported by the Beijing Academic Innovation Group in Urban Stormwater System and Water Environmental Eco-Technologies (PHR201106124) and Beijing Climate Change Response Research and Education Center, BCCRC (PXM 2013-014210-000122).