Rising Cost of Solids Handling and Disposal Necessitates a Smarter Approach

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Rising Cost of Solids Handling and Disposal Necessitates a Smarter Approach

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Wastewater treatment plant managers — both municipal and industrial — know that dealing with sludge represents the bulk of the cost of running their operation. These costs can range anywhere from 40-60% of total plant costs, depending on the size of the plant and initial wastewater characteristics.

Traditional forms of disposal, which include sending the sludge off to a landfill or else reutilizing it as fertilizer in agricultural applications, are becoming less and less feasible. Transport costs inch up just as landfills charge higher fees or, due to tighter regulations, refuse organic material altogether. While generations of farmers have relied on nutrient rich sludge as an inexpensive (or in some cases, free) method of replenishing their cropland, health regulators from Europe to America are increasingly concerned about the potential negative health impacts of contaminant uptake in the food supply. This presents a problem not just for farmers, but also for treatment plant managers who relied on the sludge-to-fertilizer route as a way to avoid disposal fees. These developments are driving the trend toward managing the entire sludge cycle — from thickening to dewatering to reutilization in the plant's energy supply — on site and in a more sustainable way. 

Each wastewater treatment plant requires a tailor-made system

When formulating the best sludge management system for a given plant, there is no silver bullet. Sludge digestion methods that aim to reduce volatile solids content consist of chemical, biological or thermal processes, the choice of which is determined by the characteristics of the feed water and the ultimate end use of the treated sludge, as well as cost and efficiency. Sludge disposal costs are minimized when the volume of sludge itself is decreased, so the best technologies will achieve reduction of sludge volume in a dependable, low maintenance and cost-effective way. 

Reducing sludge volume is a multistep process

A total sludge management system will include sludge screening, thickening, dewatering, drying, and disposal and/or reuse. At each step of the process plant managers are faced with several options to employ technology to achieve their ultimate objective. The solution selected at each step must weigh efficiency, performance and reliability alongside cost and technical considerations. 

Sludge screening filters out debris and protects the system
Before sludge can advance to either aerobic or anaerobic sludge digestion processes, it is necessary to remove any “disturbing solids” such as hair, fiber, plastics and other coarse material. This step prevents any disruption of equipment damage that could occur in subsequent treatment stages. Technologies used for sludge screening can include screw presses, screen presses and fine screening devices. 

Sludge thickening reduces sludge volume by up to 90%
Sludge thickening precedes sludge digestion and stabilization, and reduces the free water content in the sludge, leaving a biosolids content of between 5% and 10%. This initial reduction in water volume allows for a smaller and more cost-effective sludge digester. Common methods of sludge thickening include disc thickeners, screw thickeners and filter belts. Each option has its strengths and is selected based on the unique scenario of the plant and the sludge. A disc thickener is compact and its design is ideal for small footprint scenarios. The filter bed is suited to sludge with low settling properties, while screw thickeners are made of stainless steel, allowing for perfect finishing and corrosion resistance. In all cases, water use, electricity use and maintenance demand should be weighed against cost and feed water suitability. 

Sludge dewatering further reduces water content and volume
Sludge dewatering, which can occur before or after digestion, further reduces the free water content of the sludge, resulting in a solid content of between 20% and 80%. The goal in dewatering is to create a liquid stream with the least possible solids content and a solid product with the least possible water content. Common methods of sludge dewatering include screw presses, belt filter presses and centrifuges. When selecting among these options, considerations will include whether the input sludge is raw or already digested, whether it has gone through thickening, and the ultimate disposal method to be employed (landfill, energy reuse, agriculture, etc). The best technologies will be designed to handle varying sludge quality, maintenance demand, sensitivity to coarse material, and is optimized for power and water consumption.

Sludge drying produces a drier and lighter biosolids product
While dewatered sludge is suitable for agricultural applications, the increasing restrictions of using sludge as fertilizer result in transport and landfill disposal as the alternative. Because sludge volume affects transport and disposal costs, achieving a disinfected biosolids product that is both dry and granular will significantly reduce these costs. This final dry product is achieved with sludge drying, via a solar active dryer or a belt dryer, where an 80% volume reduction and 90% weight reduction is possible. When selecting a belt dryer, consideration must be taken to balance the electric heat input needed to dry the sludge with the cost and environmental sustainability of waste heat utilization. Solar dryers, in which the power of the sun is employed to dry the sludge, offer an eco-friendly alternative. 

Dry biosolids are easier to dispose and can be turned into a renewable energy source
The organic and thermal properties of dried sludge make it an asset if utilized smartly. The biogas and biomethane produced by sludge can be captured and reused in a variety of applications, including biofuel, electricity (both on site and fed into an electricity grid), and heat generation. Depending on the scenario, this power source can be used to make a wastewater treatment plant 100% energy self-sufficient, or be used as a revenue source when sold to third parties. Similarly, dewatered digested sludge can be put to use in fuel applications, composting and agriculture. 

A multifaceted, integrated approach to sludge management and disposal will lower costs and help the environment

Whether a wastewater treatment plant is municipal or industrial, large, medium or small, the approach to sludge in the future will require innovation and smart technology like never before. Environmental regulations are the primary driver in the shifting sludge management and disposal landscape. The predominant transport-to-landfill and sludge-as-fertilizer routes are diminishing. New opportunities exist in reducing sludge volume in order to minimize disposal, and putting dry biosolids to use in the energy supply and other applications. Because the sludge treatment and drying process is complex and expensive, selecting the appropriate technology at each stage is essential if plant operators are to harness these new opportunities. The best options will be tailor-made for site characteristics, feed water properties and end-use applications. Ideal technology suppliers are those that offer a wide array of products and design flexibility, and do so in an energy and cost-efficient manner.

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