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How To Power Your Treatment Plant And Reduce Costs With Waste-To-Energy Technology

Electricity is a major cost for industry operations, and the cost of meeting environmental regulations is right up there as well. At the same time, industries are compelled to improve sustainability and lower their carbon footprints. Fluence understands the constant pressures to increase efficiency and protect the environment — all while producing a great product.

Fabio Poletto of Fluence spoke with Water Online to shed some light on waste-to-energy technology (WTE). He explains how this technology can improve the environment while reducing operational costs for various industries.

In general, how does a WTE facility work, and what are its advantages?The main concept behind a WTE facility is to completely utilize the energy contained in the wastes prior to disposal.

The WTE facility converts the wastes to biogas (a mixture of methane and carbon dioxide), using a biological process called anaerobic digestion. The residual from this conversion is a “digestate” with low energy. The digestate is completely fermented and therefore stable, and very useful as fertilizer. The produced biogas is pure energy and is easy to handle. It can be used as is for electrical and thermal energy generation in an engine or in a boiler, or purified and injected

into the grid. A WTE plant has the benefit of completely closing the production cycle by utilizing all of the facility’s incoming raw materials. It’s useful to know that 1 kg of Chemical Oxygen Demand (COD), treated by an anaerobic digestion process, produces 0.35 cubic meters of methane. The same treatment made by an aerobic system requires 1 KWh of electrical energy. Another great advantage is the sludge production — in an anaerobic environment, the sludge production is only 0.05 kg per kg of treated COD. This number has to be compared with the sludge growth in an aerobic environment that is 0.45 kg per kg of treated COD. So, the advantages of a WTE facility are biogas production, low energy requirements, and low sludge production.

What type of biomass can be used as an energy resource?Generally, most of the “organic wastes” can be considered as biomasses suitable for a WTE plant. Wastes from the food and beverage processing industry are especially suitable.

Examples include: wastewater from food and beverage companies, milk whey, primary sludge from wastewater pretreatment, expired foods, manure, spent bleached earth from oil refineries, and biosolids from fermentation processes.

Is there a way to determine whether installing a WTE plant will have a good return on investment?Absolutely yes. The Fluence team has vast experience in a wide range of biomasses. Fluence can evaluate the potential biogas production, and the operative and capital costs of the plant. Working closely with the customer, they define a business plan for the entire system. In cases where the biomass is difficult to treat, Fluence has the capability to test the process in our lab. Based on the results, Fluence can guarantee the performance of the plant. For food industry waste such as that from slaughterhouses, is there a particular treatment configuration that is more effective and efficient in producing energy and treating waste?Normally the treatment configuration is not a standard scheme. It must be tailor-made on the basis of the customer’s needs and local regulations.

As a standard solution for slaughterhouses, Fluence developed a process to maximize the efficiency of the system and minimize operative cost. The wastewater coming from the factory is first treated by dissolved air flotation (DAF). This will remove 80 percent of the organic load and 65 percent of the nitrogen load. After that, the sludge is treated with an anaerobic digestion (AD) process that is able to handle very high nitrogen loads.

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(The ammonia above 3500 mg/L acts as an inhibitor of the AD process). It’s useful to know that one ton of floated sludge at 10 percent digested mass can produce up to 60 cubic meters of methane. After the AD process, the digestate is separated into solid and liquid fractions. Due to the AD process, the solid sludge to be disposed of is normally less than 20 percent of the incoming waste. The solid digestate can be used as fertilizer.

The liquid digestate is sent to the wastewater treatment plant (WWTP) with the clarified water from the initial DAF unit.

The WWTP is a nitrification-denitrification plant with a particular configuration called “double stage.” The double stage configuration can reduce the COD to below 100 mg/L and the total nitrogen to below 20 mg/L. The nitrification-denitrification plant can be equipped with a pretreatment system that uses the nitrosation process in order to reduce operational costs.

The right balance among the DAF removal efficiency, AD removal efficiency, and the capacity of the WWTP results in minimum operational and capital costs.

How does rapid anaerobic digestion work, and what is its application for breweries?The rapid anaerobic digestion technology uses the capability of the anaerobic biomass to grow as granules (like caviar) under certain conditions. The granules have a high settleability rate, which allows a high amount of biomass to remain in the reactor. Therefore, the system can handle high organic loads (up to 20 kg COD per cubic meter per day), reducing the reactor volume and, therefore, the capital cost.

Our External Forced Circulation (EFC) reactor is an evolution of an upflow anaerobic sludge blanket (UASB) reactor developed by Fluence. An increased flow velocity using effluent recycle permits partial expansion of the granular sludge bed. This improves wastewater-sludge contact. The improved contact enhances segregation of small inactive suspended particles from the sludge bed.

This kind of reactor/process is particularly suitable as pretreatment of wastewater containing soluble organic material (sugars, starches, etc). Examples include paper mills, jam producers, potato-processing factories, and breweries. The process can be applied to existing plants, reducing the organic load up to 80 percent.

In breweries, the wastewater is pretreated in a clarifier to remove the solids, then fed to the rapid anaerobic digester. This process transforms 85 percent of the total organic load into biogas. Also, solids from the first separation step can be treated in a sludge anaerobic digester. This treatment will maximize the biogas production and minimize the sludge disposal costs. Use of rapid anaerobic digestion in breweries guarantees low operative costs and a very stable process. Rapid anaerobic digestion also reduces issues related to overloading aerobic plants (especially

Chicken slaughterhouse: Sludge anaerobic digestion of primary sludge.

Chicken slaughterhouse: Retrofit of a nitrification-denitrification plant with a new aeration system, sludge anaerobic digestion. Project completed without any plant interruption.

Brewery: The EFC reactor is pictured on the right and the sludge anaerobic digester is pictured on the left

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sludge bulking).

The milk whey byproduct from the production of Greek yogurt is acidic, making treatment and disposal difficult. How can a waste-to-energy facility help to resolve this issue?The main problem with this kind of whey is the acidity that prevents recovery of valuable products like whey protein concentrate or milk powder. A WTE facility

allows recovery of the energy contained in this byproduct as biogas. At the same time, the process removes 90 percent of the organic load. The liquid digestate after the solids separation step can be easily handled by a WWTP.

As a practical example, an acid milk whey has a COD of about 60 to 65 g/L; at the outlet of the anaerobic digester (after the separation of the solids) we are able to

obtain liquid with a COD of only 500 to 600 mg/L. One cubic meter of acid milk whey can produce 20.5 cubic meters of methane.

How can a WTE plant be incorporated into an existing treatment system without disturbing the ongoing operation of the facility?Normally, a WTE plant uses the wastes that the factory disposes of or sends them to pre-treatment (in the case of an EFC Reactor) as biomass. This means that the installation of the WTE plant doesn’t affect the ongoing operation of the facility. What can affect existing operations is the retrofit of the finishing plant (nitrification-denitrification, as an example) used to treat the nitrogen load coming from the AD plant.

In that case, it’s necessary to apply technologies to avoid interference with the normal operation of the WWTP. These technologies may include extractable air diffusion systems, partitioning of tanks, and scheduling the critical activities during certain periods. The facility may need to use an equalization tank to collect the wastewater coming from the factory.

Chicken slaughterhouse: Retrofit of a nitrification/denitrification plant with a new aeration system without any interruption to the plant.

As an example, one of our customers had a WWTP, but due to increased production, wanted to reduce disposal costs while meeting EU environmental standards.

The main challenge in this case was to build and activate the WTE plant and retrofit the WWTP during the normal operation of the slaughterhouse.The slaughterhouse operates six days

Paper Mill: EFC Reactor on the left and aerobic finishing plant on right.

Dairy: Milk whey anaerobic digester for 100m3/day of milk whey able to produce 7200 KWh/day of electrical energy and 7200 KWh/day of thermal energy.

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a week, processing more than 152,000 chickens a day and generating 3,100 cubic meters of wastewaters.

Fluence designed and renovated the existing plant. The project included addition of a DAF unit, followed by an anaerobic digestion, nitrification-denitrification, and final clarification. All the construction and commissioning of the plant had to be done without any shut-down of the facility.

How is the hydrogen sulfide (H2S) in biogas managed in a WTE facility?Biogas produced in certain applications (paper mills, candied fruits producers, jam producers) is very rich in H2S, which is corrosive for boilers and cogenerators.

Usually, biogas is desulfurized with an alkaline washing in a scrubber column. This solution is very effective in terms of H2S removal, but it consumes a great amount of sodium hydroxide.

Fluence applies a washing that is similar but does not consume alkalinity. Fluence uses a synergic process that transfers H2S from the gas to the washing solution, consuming part of the alkalinity. The next step is oxidation (by air) of this solution. The result is that the aqueous H2S is

transformed into solid (and insoluble) sulphur that can be easily separated by clarification of the solution. This process restores the initial alkalinity of the solution.

The benefit is a high removal yield typical of washing scrubbers, combined with greatly reduced consumption of sodium hydroxide.

Many facilities dispose of the digestate on farmland. What can be done to keep excessive amounts of nitrogen from affecting the environment?In digestate from food and beverage wastes, the total nitrogen is normally 50 percent as bounded (proteins, struvite), and 50 percent as ammonium bicarbonate in solution.

Instead of disposing the digestate on farmland as is, the liquid can be separated and treated. Treatment can occur in a nitrification-denitrification or nitrosation plant, or in a stripping system with the recovery of ammonium phosphate. This reduces the amount of nitrogen remaining for disposal by at least 40 to 50 percent.

The nitrification process is essentially the biochemical oxidation of ammonia to nitrate, by means of nitrifying bacteria. This process takes place in a basin that’s perfectly aerated.

The nitrates produced must be reduced through a denitrification process. Bacteria oxidize the organic substances present in wastewater using oxygen molecules from nitrates. The result of this process is nitrogen in the form of gas that will be dispersed into the atmosphere.

Fluence has also developed a special “dual-stage” process that is particularly suited for very high levels of nitrogen and very low COD/TKN ratios; this situation is particularly critical and occurs frequently in slaughterhouses, during anaerobic digestion of primary sludge.

Fluence is also developing new processes, such as the nitrosation process. This process will be used for the reduction of ammonia from concentrated wastes,

such as liquid digestate. Nitrosation will further minimize the energy demand and cost of the process. It will also reduce the injection of external COD, which is sometimes needed to ensure a complete denitrification process.

Chicken slaughterhouse: Retrofit of a nitrification-denitrification plant with a new aeration system, sludge anaerobic digestion without any interruption to the plant.

The stripping system allows removal of ammonia from the digestate using air and thermal energy or an alkaline solution (caustic soda or lime). The ammonia can be recovered as a concentrated ammonium phosphate or ammonium sulphate solution and sold as fertilizer.

The choice of technologies depends on the type of system (industrial, agricultural, or municipal plant) with which it will be used. The technology should be evaluated from time to time in order to balance the capital and operative costs.

How does routine or unexpected maintenance affect operation of biogas digesters?During the engineering phases, all possible issues that can affect the stability of the process and the safety of the system are evaluated and minimized. For instance, engineers will consider the use of standby pumps for redundancy and easily maintainable mixers.

Anaerobic digestion plants, if properly sized and engineered, are very stable and are not affected by temporary shut-downs. During plant restarts, especially after some weeks of shut-down, it’s very important to pay attention to the applied organic load in order to avoid temporarily overloading the active biomass.

Pharmaceutical company: Treats up to 200 m3/h of biogas with an H2S content up to 20,000 ppm.