May 2013, Vol. 5, No.25

Research Notes

Biosolids nutrients have a long-lasting stay in soil  

Phosphorus and other minerals from land-applied biosolids can remain in the soil for many years, according to research by Eton Codling, Agricultural Research Service agronomist.  

Codling has been investigating how long plant-available phosphorus and other minerals from biosolids stay in soil and how much phosphorus, copper, cadmium, lead, manganese, and zinc is taken up by wheat growing in this soil, according to the article “The Real Dirt on Biosolids as Soil Amendments” that appeared in the January 2013 issue of Agricultural Research magazine. 

Monitoring minerals in soil  

Codling monitored mineral levels of soil that had received a single amendment of biosolids, which had been processed by either heat, lime addition, anaerobic digestion, or air drying. The biosolids had been added to the soil during another study 16 to 24 years prior to Codling’s research project. Overall, he observed higher levels of phosphorus, cadmium, copper, and zinc in the biosolids-amended soil than the control soil, the article says. 

Codling also found that phosphorus solubility in soil varied with the process used to treat the biosolids and the amount of biosolids applied. Soil with heat-treated biosolids contained higher levels of soluble phosphorus than soil amended with lime-treated biosolids, even in soil that received three times more lime-treated biosolids. This occurred because lime-treated biosolids sequestered phosphorus in low-solubility calcium phosphate compounds, the article says. 

Examining the effects on wheat  

Codling conducted another study examining wheat grown in pots containing each type of amended soil. He found that wheat yields often were higher in soils amended with biosolids than wheat grown in control soil. The highest yields came from soil containing biosolids processed by anaerobic digestion. Yields were reduced in soils with lime-treated biosolids; this likely was due to manganese deficiency, the article says.  

During this study, Codling also measured accumulated mineral levels in the above-ground biomass. Wheat grown in any of the biosolids-amended soils had higher phosphorus concentrations than wheat grown in control soils, coinciding with elevated levels of plant-available phosphorus in the soil. 

Because soil mineral levels were reduced after harvesting one wheat crop, and all material leaching from the pot after watering was collected and returned to the pots, Codling concluded that the mineral level reduction most likely resulted from plant uptake, the article says. 

  

Enhanced desalination process shows promise for developing countries and fracking site applications  

Optimization of a humidification–dehumidification (HDH) system could provide a solution for both desalination and operations treating water from natural gas wells.  

Deep water that flows out along with natural gas, called produced water, often contains more salt than oceanwater as well as other minerals. But a process developed by a team of Massachusetts Institute of Technology (MIT; Cambridge) researchers and collaborators at King Fahd University of Petroleum and Minerals (Dhahran, Saudi Arabia) may offer a less expensive and more efficient solution, according to an MIT news release.  

The process is a variation on traditional distillation where salt water is vaporized and condensed on a cold surface, separating salt from water through evaporation. But the traditional process is energy-intensive and costly because water must be heated to the boiling point while condensing surfaces must be kept cold, the news release says.  

For the new process, water that is well below boiling point is vaporized through direct contact with a carrier gas and bubbled through cooler water where purified vapor condenses, the news release says. Because the temperature difference between the warm and cool water is less than conventional dehumidifiers and the surface area provided by the bubbles is greater than that of a flat condenser surface, the process requires less energy and is more efficient.  

MIT and King Fahd University began collaborating in an effort to provide water to off-the-water-grid regions in the developing world. They sought to create a process that increased energy and thermal efficiency while reducing system size and cost, the news release says. The optimal production capacity for this HDH system is about 1200 to 2400 L/d of clean water, but its size can be increased by adding more modules, the news release says.  

The system is designed to desalinate water in developing countries. But, because the system’s efficiency is unaffected by the amount of salt in water, it also can produce clean water from hydraulic fracturing produced water.  

The team constructed a 4-m-tall (12-ft-tall) test unit that ran continuously for weeks and produced almost 700 L/d of water that met drinking water standards, the news release says. 

The team has filed to receive patents for the system and created a company to commercialize it. The team plans to scale up the system to about 2- to 3-times larger than the test unit. Prakash Narayan, MIT researcher on the team, estimates that the first commercial plants using this system could be operational within 2 years, the news release says. 


 

March WER reveals optimal dissolved oxygen for nitrification–denitrification  

Researchers from the Harbin (China) Institute of Technology found the optimal dissolved oxygen range for simultaneous nitrification–denitrification. In this process, high dissolved oxygen concentrations can cause inferior nitrogen removal because oxygen inhibits the denitrifying process, according to the research report. 

The research, published in the March 2013 issue of Water Environment Research (WER), determined that nitrate removal rates gradually diminish as dissolved oxygen concentration increases to higher than 6 mg/L. 

The researchers evaluated different effects of dissolved oxygen on simultaneous nitrification–denitrification performance in polyurethane foam contact oxidation reactors in a municipal wastewater treatment process. The study found that nitrogen removal remained optimal within the dissolved oxygen range of 0.5 to 1 mg/L, where removal efficiencies were 85.1% for ammonium and 70.6% for total nitrogen.  

The researchers also performed denaturing gradient gel electrophoresis analysis to record changes to the diversity of the microbial community as dissolved oxygen values increase. The study found that the microbial nitrifiers in reactors were Nitrosospira sp., Nitrosomonas sp., and Nitrospira sp., the report says. 

The article, “Evaluating the Effect of Dissolved Oxygen on Simultaneous Nitrification and Denitrification in Polyurethane Foam Contact Oxidation Reactors,” is available as an open-access document and can be downloaded free at http://goo.gl/77EGC.