Advanced treatment demonstration project yields valuable information

Primary treatment alternatives

In an effort to enhance the effectiveness of primary treatment, a ballasted flocculation unit uses many minute particles, which attract and hold materials together. Coagulating chemicals are also added to further clump particles together. The larger, heavier particles settle out of the wastewater rapidly. This unit may significantly improve solids and organic material removal compared to conventional primary treatments.

Wastewater flows from the bottom of this machine to the top through lots of fuzzy pink balls that pick up most of the solid particles. This technology is typically used as an advanced treatment and has not been adapted to primary treatment elsewhere. If it is successful, it could dramatically reduce the size, or footprint, of the primary treatment process.

Secondary treatment alternative

Primary treated wastewater and air are injected at the bottom of this unit. It moves to the top through lots of granules. The granules both separate solids and provide a surface for biological activity to take place. Biological Aerated Filtration can provide secondary treatment quality without taking up as much space. In addition, this technology can be adapted to function as a nitrogen removal process (nitrification-denitrification) which may be important if reclaimed water is used in the future to augment stream flow.

Advanced secondary treatment alternative

This unit combines an activated sludge secondary treatment bioreactor and a microfiltration membrane. Membranes are submerged in the aeration tank and water is drawn through the membrane with a low-pressure vacuum, leaving the solids in the aeration tank. The Membrane Bioreactor can convert screened sewage to clean effluent in a single process - eliminating the need for separate primary, secondary and advanced treatment. It produces a very high quality effluent meeting Class A criteria (after disinfection). This technology has the potential to significantly reduce plant footprint while producing improved effluent quality.

Advanced treatment alternatives

This upflow filtration process was described above as a primary treatment alternative.The porosity of the filter bed can be altered by how compressed the fuzzy pink balls (media) are. In an advanced treatment application the loading rate on a fuzzy filter can be 5 times higher than typical sand filtration. This would result in a substantially smaller footprint.

Microfiltration membranes (such as MEMBRANE BIOREACTORS) are used for physical separation of small particles from liquids. Membranes can be classified according to their pore size. There are four main types of membranes (listed from largest pore size to smallest): microfiltration, ultrafiltration, nanofiltration and reverse osmosis. Membranes are designed to operate in a pressure or vacuum mode.

In pressure membranes, the wastewater to be treated is forced down the center of the spaghetti-like membrane and is pushed through the walls in an 'inside-out' direction. For vacuum membranes, wastewater is drawn from the outside of the membrane into the hollow core where it is collected. Microfiltration membranes can be designed as strands, sheets or plates depending on the manufacturer and application.

Microfiltration membranes can be used for direct filtration of secondary effluent or as pretreatment for reverse osmosis membranes (see next page). Microfiltration has the potential to produce better effluent quality when compared to standard sand filtration technologies.

Reverse osmosis is a high-pressure membrane process capable of removing bacteria, viruses, dissolved organic matter and salts from liquids. Because of the small pore size, reverse osmosis operates most effectively on wastewater that has been subjected to microfiltration or ultrafiltration pretreatment. Particles that cannot pass through the reverse osmosis membrane are concentrated in a sidestream that must be treated or disposed. This reject stream typically amounts to 15 percent of the influent flow. Reverse osmosis produces extremely high quality water for industrial processes and potentially some reuse applications that cannot tolerate salts or dissolved organics.