Bubbly jets in stagnant water

Dissolved oxygen (DO) depression in urban rivers appears to be increasing in developing countries, which causes severe aquatic ecosystem stresses. One urban river which suffers DO depression under low flow conditions and requires systematic research for effective mitigation strategies is the Nanfei River (Hefei, China). We investigated its longitudinal profiles of DO and other related water constituents with high spatial resolution monitoring at low flow. A mechanistic DO model for the reach was customized and calibrated with the data obtained. We found that the daily average DO levels within the 11 km study reach shifted from supersaturation (11.5 mg L−1) upstream of the Wangtang Wastewater Treatment Plant (WWTP) to serious depletion (3.6 mg L−1) downstream. Process analysis indicated that DO production via strong algal photosynthesis overwhelmed the DO consumptions upstream from the WWTP. In contrast, DO sources could not compensate for DO consumptions, wherein carbonaceous deoxygenation was the largest consumer of the DO (approximately 70%) downstream the WWTP. Rather than directly contributing labile organics, the WWTP effluent affected the DO balance by shifting the metabolism from upstream autotrophy to downstream heterotrophy. Finally, mitigation strategies for DO depression in rivers in rapidly-urbanizing regions were suggested accordingly.

https://www.mdpi.com/2073-4441/9/7/520/pdf

Modelling Dissolved Oxygen Depression in an Urban River in China

Bubble characteristics of air–water bubbly jets in crossflow

Air injection systems have a wide range of environmental engineering applications. In this study, we conducted experiments on air injection in a relatively large water tank to investigate the effect of nozzle type, including single/multiple orifice nozzles and a porous airstone, on the characteristics of the bubbles and the induced flow structure. Measurements of bubble characteristics and flow field surrounding the bubble core were obtained using a double-tip optical probe and particle image velocimetry, respectively. The results revealed that bubble velocity did not change significantly with different nozzles, but bubble size decreased significantly while interfacial area, liquid entrainment rate, and kinetic energy of the mean and turbulent flow increased significantly by using the porous airstone instead of nozzles with large orifices. The results for a nozzle with multiple orifices of small diameter are comparable to those for the airstone, which suggests the suitability of its use for systems susceptible to clogging of the pores. Correlations using adequate length and velocity scales are also proposed to describe both bubble and liquid flow characteristics. Finally, applications of the results for different artificial aeration/mixing systems are presented.

Air Injection in Water with Different Nozzles

Industrial sector is the backbone of the development of a country; however, its untreated effluents are damaging various ecological phenomena. Every day huge amounts of municipal, industrial and agricultural wastes are being released into the environment, causing serious ecological problems. Pakistan is currently experiencing profound demographic, economic changes and energy crisis that pose serious limitations on spending capital on water treatment. However, no serious effort has been put forth by any agency in Pakistan to characterize the exact nature and concentrations of contaminants found in wastewaters originating from various industrial zones. The current study aimed to characterize the industrial effluent quality of Hattar Industrial Estate, Hattar, Pakistan. The effluent samples were collected from five different drains, each having the combined effluent of many industries. All the samples were characterized for pH, temperature, TS, TDS, TSS, EC, DO, BOD, COD, turbidity, nitrates (NO3–N), phosphates (PO4), ammonia (NH4–N) and different heavy metals according to the standard methods. Majority of the water quality parameters exceeded the National Environmental Quality Standards in the country. These pollutants may have serious impacts on the soil profile, groundwater quality and freshwater ecosystems and thus need attention of policy makers to install a centralized wastewater treatment plant.

/pdf/chemical-pollutants-from-an-industrial-estate-in-pakistan-a-29b1826jml.pdf

Chemical pollutants from an industrial estate in Pakistan: a threat to environmental sustainability

In ice-covered rivers, dissolved oxygen (DO) might fall below critical levels for aquatic biota in the absence of surface aeration, combined with low winter flow conditions and reduced photosynthesis rates. Open-water zones, however, can be created downstream of a diffuser by warm effluent discharges, resulting in an increase in surface aeration. In this study, we modeled the behavior of the effluent plume and the resulting open-water lead development in the Athabasca River, Alberta, Canada downstream of a pulp mill diffuser. The DO was found to increase by 0.26 mg∕L due to surface aeration of an open-water lead of 6.07 km . We also evaluated oxygen injection into the effluent pipeline to increase the DO in the river. At an injection rate of 3,500 and 5,000 lb∕day of liquid oxygen, the DO was increased by 0.16 and 0.21 mg∕L , which corresponded to an absorption efficiency of about 50%. The artificial aeration technique evaluated here appears to be an effective alternative to increase DO levels in ice-cove...

Dissolved Oxygen Downstream of an Effluent Outfall in an Ice-Covered River: Natural and Artificial Aeration

Sediment samples were collected from fifteen different locations along Athabasca River in an attempt to quantify the sediment contribution to oxygen demand. SOD rates along Athabasca River ranged from 0.09 g/m 2 ⋅d to 0.71 g/m 2 ⋅d near ambient temperature (<4°C). SOD rates generally increased downstream, however, inter-site differences were also influenced by the physical and chemical properties of the sediment. SOD rates were also higher below wastewater discharges and tributary confluences. Variation in SOD rates along the river system were positively correlated with water column concentration of TOC, TDP, Cholorphyll-α. Sediment characteristics such as grain size was a fundamental factor in determining SOD rates. Fine grained sediment in depositional areas tended to have higher SOD. The result also highlighted the importance of inclusion of sediment diagenesis and biochemical reactions inside sediment in water quality models that seek to represent water chemistry and particularly carbon, nitrogen, phosphorus and dissolved oxygen. Results from the Athabasca River suggest that SOD rates are a primary forcing function in large high order rivers that should be determined from the complex physical and chemical factors that control it rather than from water column nutrients.

Spatial Variation of Sediment Oxygen Demand in Athabasca River: Influence of Water Column Pollutants

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Dissolved Oxygen Downstream of an Effluent Outfall in an Ice-Covered River: Natural and Artificial Aeration

Spatial Variation of Sediment Oxygen Demand in Athabasca River: Influence of Water Column Pollutants