Loading of an Anaerobic Packed Bed for Treatment of a High Strength Gray Water

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ABSTRACT
In preparation of long-duration manned space flight, biological treatment systems have been evaluated for mission specific waste streams consisting of a high-strength gray water. The purpose of the work was to determine the appropriateness of using nitrification and denitrification to treat the urine-humidity condensate waste stream and to determine the optimal loading rate for the bench scale treatment system. Biological treatment of the high strength graywater was kinetically and stoichiometrically limited. Due to the low C:N ratio of this wastewater (0.85:1), biological treatment at long HRTs is stoichiometrically limited and may be overcome by adding alkalinity to improve nitrification efficiency and organic carbon to improve denitrification efficiency.

INTRODUCTION
Currently, manned space missions like the International Space Station (ISS) are not selfsufficient as far as their water supply is concerned and rely upon multiple shuttle missions for resupply. Owing to the extensive use of light-weight materials in space missions, water is one of the heaviest materials used during a mission, making shuttle resupply extremely expensive propositions, as it is estimated that 1 liter of water supplied to the International Space Station (ISS) costs $ 20,000. The cost of resupply increases in direct proportion to the duration of the mission and the payload weight, which has severe monetary implications. Current conditions emphasize the need for developing more efficient environmental control and advanced life support systems for onboard conversion of wastewater to potable water, making these long-term space missions self-sufficient. This has led to the recent emphasis on the development of waterrecycling systems for space missions.

Considerations in the design of water reclamation systems in space include shelf life, resupplyreturn logistics, crew time needed for maintenance, energy requirements to operate the system, launch weight, and stowage volume. Currently, two different processes train philosophies are under investigation for the appropriateness for long-duration space missions such as a lunar base or a trip to Mars. One approach consists of an integrated biological/physicochemical system, while the other approach consists of a purely physicochemical treatment processes.

Biological treatment has several advantages over physicochemical treatment methods (i.e., reverse osmosis, ion exchange, distillation, etc.) including: [1] less energy inputs; [2] minimal use of expendables such as ion-exchange resins and reverse osmosis membranes, reducing payload requirements; [3] minimal wastes produced that require storage and handling until final disposal; and [4] minimal chemical additions to augment treatment efficiency, further reducing payload requirements. When used in this capacity, biological treatment systems have the advantage of significantly reducing the load on downstream physicochemical treatment processes. Physiochemical systems will be required to treat the biologically treated wastewater to potable water standards; however, the size of the physiochemical units will be significantly reduced as the contaminant concentration is reduced during biological treatment.

As NASA prepares for long-duration manned space flights, the applicability of biological wastewater treatment systems must be evaluated for the waste streams anticipated during the mission. NASA has demonstrated the appropriateness of biological treatment technologies on dilute waste streams (Campbell et al., 2003a; Campbell et al., 2003b); however, a more concentrated waste stream may present a greater treatment challenge. One such waste stream is the urine-humidity condensate waste stream, which consists of urine, humidity condensate collected in the cabin of the space craft from human and machine respiration, and dilution water created during urinal flushing and oral hygiene activities. The high strength gray water has a dissolved organic concentration (DOC) and total nitrogen (TN) concentration of approximately 1100 mg/L and 1500 mg/L, respectively. The system influent pH is approximately 9. The TN of the influent is predominately ammonia-nitrogen, which may cause free ammonia toxicity problems at the high pH values. Therefore, the purpose of the work was to assess the appropriateness of using nitrification and denitrification to treat the urine-humidity condensate waste stream and to determine the optimal loading rate for the bench scale treatment system.

MATERIALS AND METHODS
The denitrification-nitrification system consists of an anaerobic packed bed (APB) and a membrane-aerated bioreactor, respectively (Figure 1). The packed bed was approximately 46 cm long with an internal diameter of 7.62 cm. The surface area of the lava rock, the support media for biofilm attachment, was 0.2 m2. The resulting working volume of the reactor was 1.1 L. The reactor was inoculated from a mixed heterotrophic culture from the TTU-WRS (Jackson and Morse, 2005).

The membrane-aerated reactor (AMR) was designed at TTU (Morse et al., 2003) to allow for bubble-less aeration of wastewater, which is necessary in microgravity environments. The reactor was 45.7 cm in length and has a diameter of 10.2 cm. The membranes were approximately three times longer than the length of the reactor and had an inner and outer diameter of 0.17 cm and 0.08 cm, respectively. The membranes were placed in a random fashion to act similarly as packing media in a packed bed reactor, resulting in membrane-membrane contact. The membrane surface area for biofilm attachment and aeration was 1.098 m2; however, approximately 25 percent of the membrane surface area was not available due to membrane-membrane contact. Thus, the assumed available surface area for microbial growth and aeration was 0.825 m2. The bottom air cavity of the reactor was pressurized with facility air to 6 psi to facilitate oxygen transport from the lumen side of the membranes to the wastewater. The reactor was inoculated from a mixed culture of nitrifying organisms from the TTU-WRS (Jackson and Morse, 2005).

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