## 5 Conclusions  
The results of the dispersant effectiveness test indicated that mixing with an intensity of approximately 150 to 200 rpm is necessary to achieve hydrocarbon dispersion of 80% or higher, using either the paddle or the helical ribbon impeller. The power levels calculated in using the helical ribbon impeller were five to ten-fold lower than that required for a similar sized paddle impeller, with similar dispersant effectiveness. The oil biodegradation tests conducted with dispersants and mixing in the same manner as the dispersant effectiveness test resulted in 50% to nearly 100% degradation over a duration of 21 to 28 days, with minor variations due to dispersant type and speed of mixing in the initial oil dispersion step. The modeling of the single component hydrocarbon dissolution from droplets and subsequent aqueous phase biodegradation indicated smaller dispersed phase droplet sizes promoted more efficient biodegradation, which supports the laboratory findings that greater mixing intensity is needed for effective dispersion, dissolution, and biodegradation.  
The preliminary research findings of improved oil dispersion with higher levels of mixing energy, and use of different impeller types (helical ribbon akin to pond mixer type device), support the idea of implementing oil sludge biodegradation in a pond environment enhanced via dispersant and petroleum-degrading microbial consortia addition. However, the greatest challenge will likely be attaining the desired higher oil dispersion levels in a medium to large pond area without the need for an unreasonably high number of mixers. Future research directions will include further investigation of pilot and small full-scale mixing and oil dispersion using an impeller of the helical ribbon type that is used in commercial wastewater pond mixers, and concentrating on the ability to disperse NAPL to suitable particle sizes with reasonable power levels, and to provide adequate oxygen levels for aerobic biodegradation at operational scales larger than the 30-L pilot employed here. Additionally, the dispersing and biodegradation tests conducted in the future should incorporate means to assess the range of oil components dispersed or biodegradable, with particular interest in the fate of the heavier hydrocarbons present in a crude sample. Future work on the modeling portion of this investigation will include multicomponent dissolution and modeling using a population balance approach to represent a distribution of particles.  
The following are available online at [[http://www.mdpi.com/2076-3417/10/5/1659/s1](http://www.mdpi.com/2076-3417/10/5/1659/s1)]([http://www.mdpi.com/2076-3417/10/5/1659/s1](http://www.mdpi.com/2076-3417/10/5/1659/s1)), Table S1. Independent values for dispersant effectiveness tests conducted in 250-mL Erlenmeyer flasks; Table S2. Power number, Power consumed and energy dissipation at various rotational speeds-Lab scale with 2.5\" paddle impeller; Table S3. Power number, Power consumed and energy dissipation at various rotational speeds-Pilot scale using 4\" paddle impeller; Table S4. Power number, Power consumed and energy dissipation at various rotational speeds-Pilot scale using 4.2\" helical ribbon impeller; Table S5. ANOVA table for fractional factorial analysis (see Equation (18) in text); Derivation for Equation (7).  
**Author Contributions:** Dispersion effectiveness and biodegradation in baffled flask, N.A.; dispersion effectiveness and biodegradation in shake flasks, M.C.; dispersion effectiveness, energy dissipation, at laboratory and pilot scale, A.S.; modeling work for xylene, T.M.; modeling work for other BTEX compounds, S.R.; supervision and writing, M.A. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Conflicts of Interest:** The authors declare no conflict of interest.