Reduction of hydrogen sulfide gas in a small wastewater collection system using sodium hydroxide.

The Kennebunk Sewer District collection system experienced H2 S-induced corrosion downstream of terminus manholes for the Wells Road and Boothby Road pumping stations. An automated odor control system using sodium hydroxide (NaOH) was developed to mitigate further corrosion. System performance was quantified by recording the [H2 S] in the terminus manholes before and after NaOH treatment. Preliminary evaluation at the Wells Road facility demonstrated significant (p < 0.001) reduction in the average [H2 S] between the treatment (4.8 ± 0.3 ppm) and control (67 ± 1.5 ppm). Permanent systems installed at both facilities in 2017 yielded similar positive results. The average [H2 S] in the Wells and Boothby Road terminus manholes reduced from 89.4 ± 1.0 to 8.0 ± 0.1 ppm and from 7.9 ± 0.2 to 0.82 ± 0.06 ppm, respectively. This work demonstrates the ability of the NaOH system presented here to minimize emission of corrosive H2 S gas in small collection systems. PRACTITIONER POINTS: Biologically-produced hydrogen sulfide (H2 S) gas corrodes sewer collection system components and results in premature asset failure. Maintaining wastewater pH above 8.5 by injecting sodium hydroxide (NaOH) minimizes H2 S emission by shifting the molar distribution of sulfur species and partially inhibiting the anaerobes that produce H2 S. The practical application of this approach may be limited to small wastewater collection systems.

Clarification of algae-laden water using electrochemical processes.

Algae contamination of surface water and drinking water supplies is a significant problem particularly in rural areas. A decentralized inexpensive technology that would effectively remove algae from water would be beneficial. Electrocoagulation (EC) combined with electroflotation (EF) as a single process (ECF) is a promising algae harvesting technique with no moving parts that may be powered using a modest array of photovoltaic panels if a low power system can be developed. Here, an ECF system was constructed to study the energy required to remove algae from a simulated drinking water supply. Results from 18 ECF experiments indicated a >95% improvement of water clarity measured by optical density (OD750) could be achieved with as little energy as 1.25 kWh m-3. The key was to find the ideal combination of gas bubbles produced by EF (G) and coagulant from the EC relative to the concentration of suspended solids (S). The ideal gas to solids (G/S) ratio for the ECF system ranged from 0.09 to 0.17. In solutions containing chloride (Cl-) ions ECF produced chlorine gas which is known to contribute to disinfection. Results suggest that ECF can efficiently remove algae and simultaneously contribute to disinfecting contaminated drinking water supplies.

Production of biodiesel and biogas from algae: a review of process train options.

Algae are an attractive biofuel feedstock because of their fast growth rates and improved land use efficiency when compared with terrestrial crops. Process train components needed to produce algal biofuels include (1) cultivation, (2) harvesting, and (3) conversion into usable fuel. This paper compares various process train options and identifies knowledge gaps presently restricting the production of algal biodiesel and algae-derived biogas. This analysis identified energy-intensive processing and the inability to cultivate large quantities of lipid-rich algal biomass as major obstacles inhibiting algal biodiesel production. Anaerobic digestion of algal biomass requires fewer process train components and occurs regardless of lipid content. In either scenario, the use of wastewater effluent as a cultivation medium seems necessary to reduce greenhouse gas emissions and maximize water use efficiency. Furthermore, anaerobically digesting algal biomass generated from low-technology wastewater treatment processes represents an appropriate technology approach to algal biofuels that is poorly investigated. Coupling these processes can improve global health by improving sanitation, while providing a cleaner burning biogas alternative to indoor biomass cooking systems typical of less-developed areas.

Improved algal harvesting using suspended air flotation.

Current methods to remove algae from a liquid medium are energy intensive and expensive. This study characterized algae contained within a wastewater oxidation pond and sought to identify a more efficient harvesting technique. Analysis of oxidation pond wastewater revealed that algae, consisting primarily of Chlorella and Scenedesmus, composed approximately 80% of the solids inventory during the study period. Results demonstrated that suspended air flotation (SAF) could harvest algae with a lower air:solids (A/S) ratio, lower energy requirements, and higher loading rates compared to dissolved air flotation (DAF) (P < 0.001). Identification of a more efficient algal harvesting system may benefit wastewater treatment plants by enabling cost effective means to reduce solids content of the final effluent. Furthermore, use of SAF to harvest commercially grown Chlorella and Scenedesmus may reduce manufacturing costs of algal-based products such as fuel, fertilizer, and fish food.