11/13/2008
Eckart V: Physico-Chemical
Life Support Subsystems
V.3 Waste Processing
Objectives
Functional Requirements: collection, treatment and/or storage/disposal of waste products including wet and dry trash, feces, food preparation wastes
Waste streams may take the form of solid, semi-solid or mixed (solid/liquid/gas)
- concentrates from water management system (e.g. brine)
- gas or solid mass with absorbed gas (e.g. LiOH, activated charcoal)
Process entails collection/segregation, fractionation, stabilization/stowage and recycling where appropriate
Requirement is highly mission duration dependent
Ultimately, in a fully closed system, all ‘waste’ from one system will be recycled as an ‘input’ to another
EPA solid waste treatment info: http://www.epa.gov/epawaste/index.htm
Waste Water treatment info: http://www.remco.com/water.htm
Interagency hazardous waste treatment web site: http://www.frtr.gov/matrix2/Preface/foreword.html
Characterized by: requirement of oxidizable substance, water content, particle size and homogeneity
Biologically decomposable liquid – hygiene water, respiration, perspiration, fecal water, urine liquid
Biologically decomposable solid – fecal solids, waste with bound water, solids from urine, hygiene water, clothes
Non-decomposable liquid – medical or experimental waste
Non-decomposable solid – metals, plastics
Metabolic gases – CO2, trace gases, methane
Non-metabolic, non-decomposable gases – material off gassing
Stabilization Methods
Condensation
Wet or dry heat
Osmotic Pressure
Freezing
Extreme pH
Metallic or organic toxins
Silver oxidation
Decomposition
Mechanical processing (fractionating - to separate a chemical compound into components, as by distillation or crystallization)
Chemical treatment (pH, extraction)
Enzymatic or catalytic treatment (hydrolysis – react with water)
Desamination? – oxidative process?
deamination - to remove an amino group, NH2,
from (an organic compound)
desalination - process of removing soluble salts from water to render it suitable for drinking (by distillation, electrodialysis, freezing, ion-exchange, or RO)
Fermentation (aerobic or anaerobic) - Any of a group of chemical reactions induced by living or nonliving ferments that split complex organic compounds into relatively simple substances
Post-processing (demineralization, filtration, RO)
Candidate Processes
Super Critical Wet Oxidation (SCWO)
Makes use of water in its supercritical state (>647K / 2.2x107 Pa) (705F / 3205 psi, or 218 atm) for destructing organic compounds by oxidation
No catalyst required
In this state, normally insoluble organic compounds become soluble, as does oxygen, which permits oxidation to occur in a single phase (unlike Wet Oxidation).
Given sufficient O2 and conditions >922K and 2.53x107 Pa, organics along with atmospheric and trace contaminant gases are completely oxidized to CO2, H2 and N2.
Inorganic salts not as soluble and precipitate from solution
Organic salts are oxidized with efficiencies >99.99% in less than one minute residence time
Heat generated can be retained in the reactor with minimal thermal loss or energy required
SWCO can produce potable water from all types of waste
Drawbacks include high temp and pressure, material corrosion, and need for post treatment to remove toxic product gases
Oxidation of dilute or concentrated waste slurry (1-10% solids) at elevated temperature and pressure
Combustion takes place in air or O2 at ~14 mPa (1.4x107 Pa or 2030 psi) and temperature of 473-573K
Byproducts primarily CO2 and H2O, detailed knowledge lacking
Forms of resultant elements (C, N, H, O and P) depend on temp and pressure
Removes 60-95% of initial carbon quantity
Dissolved metals are not oxidized and will remain in liquid effluent where they may result in corrosion and eventually accumulate in the food chain
Reduces solid waste to small
volume of sterile, non-degradable ash
(Ash - solid residue of combustion. The chemical composition of an ash depends on that of the substance burned. Wood ash contains metal carbonates (e.g., potassium carbonate) and oxides formed from metals originally compounded in the wood. Coal ash usually has a high content of minerals and is sometimes contaminated with rock; during combustion the mineral matter may become partially fused, forming cinders or clinker. Bone ash is largely made up of calcium phosphate. Seaweed ash (called kelp or varec) contains sodium carbonate, potassium carbonate, and iodine that can be extracted.)
Some byproducts can be used for plant nutrients
High O2 demand and operating temp and pressure are undesirables
Products from incomplete combustion may require post treatment catalytic oxidation, which adds complexity and potential for process consumables
Rapid exothermic oxidation of combustible elements within a fuel stock
Incineration – complete combustion in presence of excess oxygen
Proteolysis - The hydrolytic breakdown of proteins into simpler, soluble substances such as peptides and amino acids, as occurs during digestion. Under heat or pressure, but in the absence of O2
Partial pyrolysis – or starved-air combustion (SAC), incomplete combustion, due to insufficient oxygen
Wastewater solids mechanically dewatered before being sent to furnace
Accomplishes oxidation for sludge with less O2 than incineration
Stable and controllable via rate of gas input
Produces relatively few toxic byproducts and particulates are larger (easier to remove)
Converts nitrogen to ammonia which is converted to N2 and H2O in the afterburner
Dry incineration – combustion of concentrated solid waste feed (feces, urine and non-human waste) evaporated to ~50% solid content
Heated to ~813K in air or O2 (preferred) at ambient pressure
End products are sterile, water condensate, inorganic ash and gas (primarily CO2)
Catalytic afterburner may be required
However, even with AB, combustion is incomplete, water may require additional treatment and energy intensive evaporation or pre-drying required
Yields nitrate (NO3), which is good for plants and nitrite (NO2), which is a phytotoxin
(in contrast, N2 is safe for plants, but largely unusable except for e.g. N2 fixing legumes)
Non-thermal electrolysis process that degrades organic solid wastes and urine by oxidation using catalytic electrodes to yield CO2, N2, O2 and H2
Does not consume O2 or require lots of power, but TRL currently low
High temperature process (920K) using catalytic oxidation
May be most efficient microbial control process
Original design used plutonium heat source - Radio Isotope Thermal Energy (RITE) System
Composting, bioprocessing, activated sludge, etc.
Time and complexity factors…
Aerobic - Bacteria operates with oxygen from photosynthesis, breaking down soluble materials as well as buffering gases generated by the anaerobic bacteria.
Anaerobic - Digests all biosolids without oxygen. Gives off methane, hydrogen-sulfide, ammonia and carbon dioxide.
Facultative - Thin zone between other two. Bacteria operate with or without oxygen. With oxygen present, will produce acids as food/stimuli for anaerobic bacteria enhancing sludge digestion.
Summary
First, identify the relevant waste streams and specific collection mechanism(s) (i.e., toilet, trashcan, atmosphere, system output, etc.) in the spacecraft
Identify list of biggest payback items based on mission duration factors
Next, characterize the candidate waste system I/O’s, including system consumables (oxygen, catalysts, chemicals, energy, etc.) and byproducts
Then consider the mass cost/benefit ratio for reclaiming waste vs. store, dump, etc., including the waste products and the system itself, along with processing time / buffering requirements
What do we ultimately do with waste on the moon or Mars?
Bottom Line(s)
What are the (likely) categories and quantities of waste products associated with each mission element?
Requirements (recommendations) for storing/disposing or recycling during each element
Consider “inter-vehicle” concepts for temporary stowage, subsequent transfer and ultimate recycling
What technologies options exist for all of the above?
What are the pros/cons of each candidate technology identified?
Other issues? (contamination of planetary surface…)