Talk:Sewage sludge treatment
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--Alex 08:54, 28 July 2006 (UTC)
[edit] sludge vs. biosolids and disposal
I think there needs to be a distinction made between biosolids and sludge. In an activated sludge process, the solids wasted out of the process are biosolids, the colonies of bacteria that eat waste matter). In a standard process with primary and secondary clarification, the resulting solids are literally sludge, whatever settles out is what you get.
With both processes, the biosolids and sludges are usually digested, anaerobically or aerobically. This is to reduce VOC content and also further the breakdown -or- digestion of organic matter. Digested biosolids from an activated sludge process are (usually) 12-15 days old, and are nothing but nutrients (phosphorus, nitrogen, ammonia, zinc, potassium, sulphur, manganese, magnesium, iron, copper, calcium, and a host of other yummy ingredients, including trace amounts of heavy metals and other pollutants). The quality and constituents of digested biosolids largely depends of the community the wastewater was collected from. For example, in a residential suburb, you may derive a very high quality biosolids that can easily be qualified for land application, but in a heavy industrial and urban community, your biosolids may contain much higher concentrations of chemicals and heavy metals, disqualifying their use as a fertilizer. The level of industrial pre-treatment also plays a large role in the quality of biosolids.
Also, a reason as to why some plants use anaerobic digestion and aerobic digestion needs to be made. For example, the capital cost of an anaerobic system is vastly greater than the cost of an aerobic system, but in the long term, the anaerobic system is more cost efficient. Aerobic systems require air pumped into them, needing power hungry blowers.
After digestion I think there needs to be more descriptions of the dewatering processes, such as:
Sludge Drying Beds Drying beds consist of long skinny concrete troughs filled with sand, and have an underdrain system. Sludge is allowed to fill the bed, and the water slowly drains through the sand, leaving the solids on top. The sludge may be left to air dry, sometimes for days, resulting in a solid that can be ~80% solids or higher. The purpose of the troughs being skinny is so that a front-end loader drive down the trough and scoop up the dried sludge, leaving the sand intact for the next batch. The tires of the loader sit on the very short concrete walls between the troughs. Drying beds are a slow dewatering process, and are perfect for plants processing less than 4MGD.
Belt Filter Press A Belt Filter Press is a carryover from the paper pulp industry. basically, it is a machine that has two belt loops that come together at one point, squeezing the water from the sludge. The solids remain between the belts and are scraped off, usually onto a conveyor. A press feature three main zones, the gravity zone, where the sludge is dropped onto the belt, and a majority of the water is drained via gravity through the belt. The shear zone, where the two belts come together and usually the initial drum that the belts travel around after the gravity zone. A lot of water is squeezed out here. Next is the pressure zone where the belts travel through a series of rollers, placing the belts under extreme pressure. This forces out the last amount of water. Belt Filter Presses are the workhorses of the dewatering section of most plants because of their ease of use, year round ability to dewater, and relatively low power consumption. The dewatered "cake" scraped off the belts is usually a wet 10% to spongy 18% solids, dry enough to haul in a dump truck.
Plate Filter Press Similar to a Belt Filter Press, except higher pressures can be obtained and thus, a drier cake in the 25-35% range. This is a slower process than the Belt Filter Press as the Plate Filter Press cannot continually process biosolids. The plate filters must be filled, squeezed together then unloaded.
Centrifuge A centrifuge is also a carryover, but from multiple industries (mainly food!) and tailored for sludge. basically, there is a large spinning drum, which may reach speeds as high as 20k RPM. Inside, there is a screw which turns at a slightly lower speed than the drum, pushing the driest solids to one end for discharge, and the liquid flows out the drum into the casing for dishcharge. Centrifuges can handle higher flow rates than comparable Belt Filter Presses, and also have drier cake, usually a spongy 18% to crumbly 26-40% solids. Centrifuges consume much more power than a belt filter press, but the savings are seen in lowered hauling costs.
Liquid Usually only in the smallest plants, sludge is hauled by tanker truck to another facility for dewatering, or some other direct use.
And even after dewatering, you have the disposal methods.
Landfill Many plants dispose of their dewatered sludge in sanitary landfills (in the U.S.). This option does have its costs, but sometimes there is no other alternative.
Land Application Depending on the quality of sludge, the dewatered sludge or biosolids can be used as a fertilizer on crops not intended for human consumption. This is by far the cheapest option, as most municipalites give it away to farmers and some even spread it themselves on the farmers fields at no charge. In the U.S. biosolids must meet "Class B" criteria for land application, which has to do with the levels of contaminates in the biosolids.
Deep Well Injection Rarely practiced, liquid sludge may be injected deep into the earth.
Given away or sold to consumers For lawn and garden use. Very stringently drawn rules and regulations govern this practice, ans the only biosolids qualified for this are "Class A" and have had their pathogenic content reduced to near nothing. "Class A" biosolids are made by cooking the "Class B" biosolids cake from the dewatering process. This is accomplished trhough a variety of means, mainly gas-fired rotating drum ovens, microwaves, and sunlight. The prohibitive cost of this makes it unattainable by most municipalities.
Ocean Dumping This practice is now illegal in the U.S. and U.S. waters, not sure about the world.
Incineration Fairly expensive because of the fuel requirement, but incineration reduces biosolids down to about 10% of its original mass and volume, making the ash very cheap to landfill. This is an option when the sludge does not meet the quality requirements for land application or even direct landfilling.
Please feel free to tear this apart. Thanks JAK83 17:37, 5 February 2007 (UTC)
- Hi. I made the following remark at Biosolids. (Copied from biosolids talk) Not a bad idea. I would rename sewage sludge treatment to just sewage sludge then merge in sludge and make sludge a disamb to include industrial sludge, water treatment sludge, etc. I would leave biosolids as a separate article due to the political and social aspects it has that are not shared by sludges that are not distributed as CFR 503 biosolids. --Justanother 04:41, 10 February 2007 (UTC)
- Good work here! I may have a few more comments later but for now let me present a different distinction in terms. Although there is not uniformity nationwide, the USEPA seems to make the distinction that sewage sludge that has been further processed for beneficial reuse under CFR Part 503 would be called "biosolids". I think it would serve us well if we went that way. I think that the organic pre-digester treatment sludges you term "biosolids" above could as well be called "biomass" and avoid confusion. How does that sound? --Justanother 06:06, 10 February 2007 (UTC)
