Byproducts of sewage treatment
The two by-products of sewage treatment are recycled wastewater and biosolids. Several treatment steps (sedimentation, digestion, disinfection with chlorine and ultraviolet light, and filtration) assure that clean water can be returned for use in manufacturing, agriculture, and even as potable water. Biosolids are essentially all the material that remains: a low solids effluent and a solids component known as sewage sludges.
Fertilizer in a field
Disposing of the volumes of sewage sludge is expensive but necessary. Landfill disposal is widely recognized as unsustainable due to pollution concerns. Historically, sewage sludge could be incinerated, but it is only a means of minimization, as dry solids—including heavy metals—remain in the ash, although there are opportunities for recovering ash for use in construction materials or a fuel in cement production, where it becomes an integral part of the product.
Fertilizer from sewage sludge
One of the most valuable byproducts from sewage sludge is fertilizer. Rich in nutrients that are essential for plant production such as nitrogen, potassium and phosphorus, biosolid compost are now distributed by wastewater plants for use on agricultural fields and even commercial compost for home gardening. While their use in agriculture has stirred some controversy, biosolids have been shown to produce yields equal to commercial fertilizer, while also improving soil chemical and biological health. An analysis of soil samples from fields fertilized with biosolids revealed high efficiency of soil carbon recovery in a dryland wheat cropping system, and that a chemically stable fraction of organic matter is accumulating in the soil.
Biosolids rules and regulations
The US federal biosolids rule is contained in 40 CFR Part 503. Biosolids that are to be land applied must meet these strict regulations and quality standards. The Part 503 rule governing the use and disposal of biosolids contain numerical limits, for metals in biosolids, pathogen reduction standards, and additional requirements for land applied biosolids.
In Europe, the EU Sewage Sludge Directive 86/278/EEC seeks to encourage the use of sewage sludge in agriculture and to regulate its use in such a way as to prevent harmful effects on soil, vegetation, animals and man. However, regulations and safety concerns are still being debated within the organization. The US Organic Materials Review Institute (OMRI), which lists products such as fertilizers, pest controls and livestock healthcare products suitable for organic food production, will not list fertilizers made from sewage sludge.
Biosolid production during sewage treatment
Biosolids are a solid product from sewage treatment processes and must be treated in order to make them safe for further use. Simply dewatering the material in order to make fertilizer is not viable. Wastewater plants have some controls over the water that they receive, particularly industrial wastewater, but certainly not complete control. Before biosolids can be returned for use, pathogens, organic pollutants such as PCBs, and heavy metals must be removed. While pathogens can be easily remediated using lime, heat and time, removing organic compounds from water requires adsorption, oxidation and filtration techniques.
Heavy metals in biosolids
In order to efficiently remove heavy metals such as cadmium, chromium, copper, arsenic and lead from industrial wastewater and biosolids, innovative physico-chemical processes are being developed, including adsorption on new adsorbents, membrane filtration, electrodialysis, and photocatalysis. According to recent studies promising methods to treat such complex systems appear to be the photocatalytic methods, which consume cheap photons from the UV-near visible region. Bioleaching techniques using species of bacteria (acidithiobacillus ferrooxidans) are being investigated as a means of recovering heavy metals from sewage sludge as a form of bioremediation.
Operators who produce and test the biosolids are required to certify that the biosolids have been treated and tested and meet regulatory standards. Improper certification can lead to large fines and jail time. While several analytical techniques are available to detect heavy metals in sludges, X-ray Fluorescence Spectroscopy provides easy access to qualitative and quantitative data.
EDXRF for sewage sludge screening
In simple terms, X-ray Fluorescence uses an energy source to excite atoms within a material. As each element has a unique electron configuration, reading the X-ray energy levels generated from the excitation phenomena (the rejection and recovery of electrons orbiting the atom) the analyst can identify the elements within the sample, and by reading the intensity of the energies know their quantities. The technique offers minimal sample preparation and non-destructive analysis. Energy-dispersive XRF (EDXRF) relies on a highly-sensitive X-ray detector to measure the emission lines of all elements from sodium (Na, Z=11) to uranium (U, Z=92) at concentrations of a few parts-per-million to a percentage of weight-to-weight proportion. A complete sample analysis takes less than 15 minutes, enabling a fast and efficient technique to screen biosolids and sewage sludge.
In order to demonstrate the utility, accuracy and ease-of-use of Energy-dispersive X-ray Fluorescence, scientists at Thermo Fisher Scientific employed a Thermo Scientific™ ARL™ QUANT’X EDXRF Spectrometer equipped with a Silicon Drift X-ray Detector (SDD) and a 50-watt Rh target X-ray tube. Using a set of nine primary beam filters designed to optimize the peak-to-background signals for all elements, the scientists studied 12 metallic elements against BRC and NIST reference materials. The ARL QUANT’X spectrometer uses a 10-position auto-sampler with spinner that allows for unattended analysis of multiple samples, making it ideal for busy labs at wastewater companies, and municipal and regulatory organizations.
The results of this study can be read in Application Note 41956, “Analysis of heavy metals in sewage sludge with ARL QUANT’X EDXRF Spectrometer.”
Further reading: What is Milorganite?
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