Graduation Year

2021

Document Type

Thesis

Degree

M.S.C.E.

Degree Name

MS in Civil Engineering (M.S.C.E.)

Degree Granting Department

Civil and Environmental Engineering

Major Professor

Sarina Ergas, Ph.D.

Co-Major Professor

Maya Trotz, Ph.D.

Committee Member

Qiong Zhang, Ph.D.

Keywords

Biological nitrogen removal, Oyster shell media, Recirculation, Salt water, Septic systems

Abstract

The infrastructure to support the growing world population has polluted many water bodies with nitrogen from urban and agricultural runoff and failing wastewater systems, such as improperly sited and maintained septic systems. Septic systems present a low energy and maintenance option to treat domestic wastewater and is a treatment option that can be done far from a centralized grid. This further reduces cost, maintenance, and energy as it reduces the need to build and maintaining sewage piping and the infrastructure to move the water longer distances. However, these systems tend to fail when their upkeep is left to homeowners that may not realize they even have a septic system or that the septic tank needs to be evacuated of sludge every three to five years. One of the biggest shortcomings in these systems and their ability to remove nutrients from wastewater. This failure has more to do with the system design, as a conventional septic system does not have a designated nutrient removal step.

A growing global population and climate change has placed stress on resources such as water and energy. The practicality of using potable water for toilet flushing has come into question as it wastes fresh water and the energy used to treat and convey it. One solution for coastal communities to reduce freshwater and energy stress for wastewater treatment is to use more readily accessible seawater for toilet flushing. Many wastewater treatment processes use microbes to breakdown contaminants, and though halophiles thrive in high salt concentrations, most microbes involved in wastewater treatment are effected negatively by high salt concentrations.

This research evaluated wastewater treatment performance of a Biological Nitrogen Removal (BNR) system for a passive Onsite Wastewater Treatments System (OWTS) using seawater for toilet flushing. The research focused on nitrogen removal and transformations occurring in a septic tank and a nitrification trickling filter using oyster shell media as both a solid phase source of alkalinity addition and a surface for microbial growth of nitrifying bacteria. Two nitrification biofilters were constructed with both oyster shell and Lightweight Expanded Clay Aggregate (LECA) media at different volume percent ratios. Biofilter 1 was 50% oyster shell to 50% LECA by volume and biofilter 2 was 17% oyster shells to 83% LECA by volume. Biofilter performance was evaluated in two phases. Phase 1 was performed with a single pass treatment train and phase 2 was performed with a 0.5:1 recirculation ratio. Nitrified effluent was recirculated to the septic tank to promote pre-denitrification.

For objective one in comparing the performance of Biofilter 1 with 50% oyster shell media and Biofilter 2 with 17% oyster shell media, results showed no significant difference in the nitrogen removal performance of the two biofilters. Biofilter 1 removed 15% TN and 85% TAN while biofilter 2 removed 13% TN and 86% TAN. In phase 2, biofilter 1 removed 75% TAN while biofilter 2 removed 82% TAN. Both systems in phase 2 removed 36% TN. The high alkalinity already presents in the wastewater for both systems made the additional alkalinity in biofilter 1 less of a factor. Objective 2 examined the effects of recirculation on nitrogen performance for the biofilters. Phase 1 with no recirculation showed slightly better nitrification in both biofilters but an average decrease of 12% TN. For Objective 3, comparing this system with prior biological nitrogen removal systems, the 15 ppt salinity did not have a noticeable effect on the systems nitrogen conversion and removal performance. For future work, the salinity should be increased to 30 ppt to further examine salinity effects. A post nitrification denitrification step should also be a subject for future work. Low COD concentrations in the biofilters nitrified effluent suggested the need for an electron donor for such a denitrification step. The best options for media addition as an electron donor for denitrification in saline wastewater based on sustainable resources available in areas where these systems are found should also be examined.

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