Pictures of the Future    Spring 2007

Solutions You Can Swallow

Maryland, Pennsylvania, Virginia and Washington D.C. are facing a huge challenge. By 2010 they intend to reduce the amount of nitrogen and phosphorus discharged into Chesapeake Bay by 50,000 t per year. These nutrients pour into the bay from nearby cities, fields and sewage treatment plants, creating extensive algal blooms, especially in the spring. When the algae die, bacteria consume not only the dead plankton, but also the oxygen in the water, transforming large parts of the 300-km-long estuary into dead zones.

In response, the three states and the District of Columbia have introduced strict limits for discharges. Starting in 2010, one liter of treated wastewater can contain an annual average of only three milligrams (mg) per liter of total nitrogen and 0.3 mg per liter of total phosphorus. "That’s near the limit of what can be achieved using biological processes to remove nitrogen, and even below the limit for phosphorus," says James Scott of Siemens Water Technologies in Salt Lake City, who grew up on Chesapeake Bay. To meet this challenge, the 66 largest municipal sewage treatment plants in Maryland are being modernized—in many cases with Siemens technology. "These measures will enable Maryland to achieve more than one-third of its reduction-target obligation," says Susan McCorvey of Water Technologies. Some older facilities will have to be rebuilt, while more modern plants will only need to be equipped with additional filters. "Proven technologies are used to remove nutrients, but these technologies now have to be refined," explains Scott.

Nitrogen and phosphorus end up in sewage because they are present in food, cleansers and human excreta. Biological treatment at a wastewater facility involves using aerobic bacteria to convert them into inorganic nitrate and phosphate. Phosphate can be precipitated using metal salts or reduced to a level of about one mg per liter together with nitrate, by means of a biological process. "Nitrate-reducing bacteria, which live under anaerobic conditions, can convert nitrate into gaseous nitrogen," says McCorvey.

Until 2003, the sewage treatment plant in Fruitland, Maryland couldn’t even remove nutrients from wastewater. But when its old filter was replaced with an OMNIFLO Sequencing Batch Reactor from Siemens everything changed. In the reactor, wastewater sequentially undergoes purification stages in the same tank. Studies show that this enables the precise control of the denitrification process using the duration of the aerobic and anaerobic stages. As a result, the nitrogen content at the Fruitland plant has been reduced by 80 %, to three mg per liter. Similarly, the town of Aberdeen, Maryland, has installed an Astrasand filter. Here, filtered wastewater flows through a movable bed of sand to remove suspended matter, and methanol is added to promote the growth of nitrate-reducing bacteria in the sand. Using a sophisticated control mechanism that regulates the dwell time of the wastewater, the purification cycles and the addition of methanol to feed the bacteria, Siemens engineers have succeeded in eliminating up to 98 % of the nitrate. "The Astrasand filter also can be used to precipitate phosphate," says McCorvey.

Radicals at Work. Microbes, however, cannot break down all organic materials. Some substances consume so much oxygen that aerobic bacteria suffocate. Now, Dr. Manfred Waidhas and his team from Corporate Technology (CT) in Erlangen have developed an electrochemical process that breaks down previously non-biodegradable molecules so that they can be split by bacteria into CO₂ and water. Electrolysis creates hydroxyl radicals in the water. These radicals are very reactive, consisting of one hydrogen atom and one oxygen atom.

The new process is being tested at a paper factory, where, every day, a membrane filter removes 600 m³ of concentrate containing lignin, which biodegrades very slowly. Until now, the concentrate has been precipitated using lime or an aluminum salt and then incinerated. "But that just shifts the problem from water to land," says Waidhas. "It’s much better to destroy the pollutant." His team is striving to break down the lignin molecules so that bacteria can consume them, using as little energy as possible. "Our process is more efficient than the alternative of adding ozone," he says.

Just Add Salt. Electrochemical processes are ideal for substances with high oxygen demand, metal compounds and dyes. However, some pollutants—including oils, pesticides, hormones and residues from pharmaceutical products—resist such methods. These can be broken down by a plasma process developed by engineers working under Dr. Werner Hartmann at CT in Erlangen (see "Elements of Life – Trends" in Pictures of the Future, Spring 2005). Here, water is exposed to high-voltage discharges, leading to the creation of radicals that can break even very stable bonds. Hartmann believes this process is well-suited for industrial wastewater applications and low concentrations of very hazardous substances.

One advantage of such processes is that they eliminate the need for chemicals. Still, chlorine gas will likely remain the standard for economically disinfecting drinking water, although, according to Alberto Garibi of Siemens Water Technologies in Miami, "Regulations are increasingly deterring water management companies from using chlorine, due to safety concerns." Sodium hypochlorite solutions are often used as a substitute because they eliminate the risk of a toxic gas release, and have the same effect as chlorine. "Sodium hypochlorite costs two to three times as much as chlorine, however," says Garibi. "And due to its high concentration, it must be stored and handled as a hazardous material. It also decomposes when stored for long periods."

The new Osec B-Pak system from Water Technologies enables the production of hypochlorite on site. This compact system is practically maintenance-free and comes with an intelligent control system for automatic operation. "All you need is salt," says Garibi. Electrolysis converts brine into sodium hypochlorite and hydrogen gas. To safely dispose of the hydrogen, it is diluted and ventilated to the atmosphere. "The hydrogen could be used in a fuel cell," says Garibi. "But recovering it isn’t yet economically feasible."
                                                                                                                         Ute Kehse