The U.S. Environmental Protection Agency’s (EPA) Water Infrastructure Resiliency and Finance Center, in collaboration with the ...
Increased regulatory pressures and demand for reused water have stimulated the development of several new wastewater technologies in the last decade. Advanced treatment systems are characterized by a small footprint, high contaminant removal efficiency, flexibility, resilience to changes in wastewater quality and high level of automation. To recycle or reuse wastewater, most contaminants have to be removed. Primary systems are used to remove suspended and dispersed solids, free and emulsified oils, fats and grease. Secondary systems, or biological treatment, are used to reduce organic contaminants. Tertiary systems such as filtration and membrane filtration are used to remove what is left after secondary treatment (i.e., ions, small nonbiodegradable molecules and very fine solids).
Several hybrid systems combining two or three different technologies recently have been developed and applied. For instance, membrane bioreactors (MBRs) are composed of biological units (aeration and microbial growth) responsible for the biodegradation of the waste compounds and a membrane filtration module for the separation of produced biosolids. Membranes can be immersed in the aeration unit or used externally. Cleaning of the membranes is achieved through frequent permeate backpulsing and occasional chemical washing. MBRs have been pilot tested and used in industrial and municipal wastewater treatment. Another configuration under development is fixed-film bioreactors, such as moving-bed biofilm reactors (MBBRs), followed by external membrane ultrafiltration (UF).
An MBBR is a hybrid of activated sludge and biofilter processes. Contrary to most fixed-film bioreactors, MBBRs utilize the whole tank volume for biomass. Contrary, however, to activated sludge reactors, MBBRs do not need return activated sludge; this is achieved by having a biomass grow on plastic high-surface-area carriers that move freely in the water volume of the reactor, kept within the reactor volume by a sieve arrangement at the reactor outlet. At the bottom of the tank, a large bubble aeration system assures mixing and floating of plastic carriers with attached biomass.
The biofilm carrier is made of high-density polyethylene (0.95 per cubic cm) and shaped like small cylinders, with a cross on the inside of the cylinder and “fins” on the outside. The original cylinders had a length of 7 mm and diameter of 10 mm. Later, various shapes and sizes were introduced by numerous manufacturers.
One of the important advantages of the MBBR is that the filling fraction of carrier in the reactor may be subject to needs. That means that by increasing the filling fraction, one can increase surface area and capacity of the reactor to reduce biological oxygen demand (BOD) without additional tanks. Microorganisms growing on such media are much more resistant to pH and toxic shock as well as fluctuations in BODs. Produced biosolids are easy to separate and dewater.
Anaerobic mobile film technology (MFT) bioreactors can be used with UF, replacing sedimentation for biosolids removal. While all of the aforementioned bioreactors do perform as advanced treatment systems, using membranes for biosolids separation still presents some challenges. Backpulsing to clean membranes requires a lot of energy, and membranes are more expensive than clarifiers or flotation systems for biosolids removal. Membrane cleaning solutions also add to the cost of treatment.
To reduce backpulsing energy needs and cleaning solutions costs, suspended solids and fats, oils and grease (FOG) should be removed prior to bioreactor or membrane treatment. FOGs are highly hydrophobic and deposit very strongly on membrane surfaces. They also have a tendency to float on top of bioreactors, reducing aeration efficiency or coating fixed-film bioreactors’ media. Removing FOGs significantly reduces foaming potential and BODs that are difficult for microorganisms to remove in any bioreactors.
Pretreatment systems can be used to neutralize pH and remove any biocides that would interfere with microorganisms’ performance. Pretreatment systems that are used ahead of advanced secondary and tertiary technologies should have a small footprint, high contaminant-removal efficiency, flexibility and design that is easy to automate and incorporate with downstream technologies.
FOGs have a density lower than that of water and are consequently removed with flotation technologies. In flotation systems, fine air bubbles are introduced and carry particles and FOGs to the surface, where they are skimmed and separated from wastewater. To remove small particles present in the wastewater, small bubbles perform best. Dissolved-air flotation (DAF) units produce the smallest bubbles, which rise to the surface very slowly. Therefore, DAF units have a large footprint (tank size). Induced-air systems produce larger bubbles with faster raise time, but they also lower performance in fine total suspended solids (TSS) removal.
Primary Treatment Systems
A hybrid centrifugal DAF system such as the GEM system combines benefits of small bubbles and fast rise time. At the heart of the system is a liquid hydro-cyclone column with heads that permit adjustments of treatment chemicals, particles and bubbles-mixing energy. Coagulants and flocculants are introduced on the top of the column at the same time with air and wastewater particles. Therefore, flocs and bubbles nucleate at the same time, producing porous sludge filled with air. Large flocs (often 1-in. diameter) are produced. Solid/liquid separation occurs inside the hydro-cyclone column. Tanks are used only for sludge skimming.
Such design results in a footprint that is only 15% to 40% that of DAF systems. Efficient mixing of treatment chemicals and particles results in very low residual amounts of chemicals such as flocculants in wastewater. This is particularly important to protect membranes and microorganisms from damage that can happen when cationic reagents adsorb. A schematic presentation of the GEM system is shown in Figure 1.
Successful primary treatment is a combination of the best system and best treatment chemicals for the system. Clean Water Technology, Inc. tested the efficiency of hundreds of coagulants and flocculants before choosing a handful that perform best, allowing the GEM system to achieve the most efficient primary treatment. A dual flocculant approach results in production of the best sludge with very high solids loading (10% to 30% by weight), low TSS and FOG in effluent, fast response and absence of cationic flocculant overdose in the effluent.
High-molecular-weight, high-charge cationic flocculants are used to neutralize charge and overcharge particles lightly. Medium-charge, ultrahigh-molecular-weight anionic flocculants are then added to grow large stable flocs and precipitate any excess cationic flocculant present. If coagulants are used to lower the cost of treatment, low-molecular-weight epi-amine or polyaluminum chloride coagulants produce the best treatment results with the lowest possible danger to microorganisms and membranes downstream.
In conclusion, no matter how efficient secondary (bioreactors) or tertiary treatments are, it makes sense to remove suspended solids and FOGs before other treatment steps. When using the new generation of advanced secondary and tertiary treatment systems, it is a good idea to try novel systems developed recently for primary treatment. A step-wise approach (i.e., screens, coagulation, flocculation, flotation, secondary treatment and tertiary treatment) delivers the most reliable treatment results at minimum cost. All-in-one systems are still the technology of the future, in spite of the great promise of MBRs and MBBRs.