Innovative Use of Dissolved Air Flotation with Biosorption as Primary Treatment to Approach Energy Neutrality in WWTPs
This paper presents two types of DAF application together with biosorption (the Captivator® system) as primary treatments. In the first instance, the Captivator system is the sole primary treatment for a new plant installation and helps to gain 65% more biogas while requiring only 44% of aeration for COD oxidation, compared to a conventional process with a primary clarifier. In the second application, the Captivator system is used to enhance the existing primary treatment for plant capacity expansion. With digested anaerobic sludge recycled as a n additional adsorbent, the Captivator system in the second application increases the biogas yield by 52% an d only generates 59% excess sludge. Overall, the Captivator system would help WWTPs to approach energy neutrality by diverting more organics for biogas production and reducing the energy requirements for aeration. In addition, it would help to reduce the installation footprint for primary treatment and save considerable capital cost by eliminating the sludge thickening process.
Startup and Performance of the World’s first Large Scale Primary Dissolved Air Floatation Clarifier
The Pima County Regional Wastewater Reclamation Department, in Tucson, Ariz., built the new 120 MLD (32 MGD) Agua Nueva Water Reclamation Facility (ANWRF) and started it up in early 2014. This facility includes the first large scale Dissolved Air Floatation (DAF) primary clarifiers in the world. DAF primary clarifiers were chosen for their benefits of low cost, small footprint, improved TSS and BOD removal (as compared to no primary treatment). They also eliminated the need for separate solids thickening, and improved oil and grease removal. However, the risk was that there was no full scale implementations in wastewater that could be used to validate the technology.
The DAF primary clarifiers at ANWRF treat both the screened raw sewage and the waste activated solids that are combined at the head of each DAF unit. The DAFs have been found to remove between 50% and 75% of the suspended material, depending on chemical addition, and between 30% and 50% of the influent COD, while reliably producing a thickened solids (primary and WAS) approaching 4 percent. Some removal of soluble organics has also been observed, likely a result of a combination of colloidal coagulation and soluble uptake by the cothickened waste activated sludge.
Primary DAF clarification at ANWRF has shown performance similar to conventional primary clarification while combining the function of WAS/Primary sludge thickening and grit removal in a single unit process. Actual operation of the DAF clarifiers has met the goals originally envisioned in the design, but with slightly higher effluent TSS values than hoped for. Continuing optimization at the facility appears to be improving overall TSS removal at lower chemical usage rates.
SHIFTING THE ENERGY BALANCE WITH BIOLOGICALLY ENHANCED PRIMARY TREATMENT -- HOW CARBON DIVERSION MAKES SENSE
A novel biologically enhanced primary treatment (BEPT) has gained rapid acceptance among carbon diversion technologies. The Captivator® system works based on the principles of biosorption to divert more organics to anaerobic digestion in lieu of aerobic oxidation. This diversion is capable of increasing biogas production by more than 40% while simultaneously reducing the amount of carbon oxidized in the mainstream activated sludge process by 40%. This BEPT system has been shown to be a unique and cost-effective alternate to other carbon diversion systems, such as chemically-enhanced primary treatment or mechanically-enhanced primary treatment, being able to extract not just higher levels of particulate BOD but also extracting a substantial amount of soluble BOD by biological means. The Captivator system has been validated though piloting and full-scale tests, proving the following performance: 50-60% BOD and 70-80% TSS removals without chemicals. It has also shown the ability to generate 4-6% thickened sludge without the need for polymers. This BEPT technology includes the use of well-established equipment in the wastewater industry arranged in an innovative configuration to achieve carbon diversion. The world’s first full-scale Captivator system came online at the Agua Nueva Water Reclamation Facility, Az., in January 2014. Additional full-scale tests will be conducted in 2016 to keep developing the modelling of biosorption kinetics of this BEPT technology.