Conceptual design of a bioregenerative life support system containing crops and silkworms

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Advances in Space Research 45 (2010) 929–939 www.elsevier.com/locate/asr

Conceptual design of a bioregenerative life support system containing crops and silkworms Enzhu Hu a, Sergey I. Bartsev b, Hong Liu a,* a

Lab of Environmental Biology and Life Support Technology, School of Biological Science and Medical Engineering, Beihang University, Beijing 100191, China b Institute of Biophysics, SB RAS, Krasnoyarsk 660036, Russia Received 12 January 2009; received in revised form 19 October 2009; accepted 29 November 2009

Abstract This article summarizes a conceptual design of a bioregenerative life support system for permanent lunar base or planetary exploration. The system consists of seven compartments – higher plants cultivation, animal rearing, human habitation, water recovery, waste treatment, atmosphere management, and storages. Fifteen kinds of crops, such as wheat, rice, soybean, lettuce, and mulberry, were selected as main life support contributors to provide the crew with air, water, and vegetable food. Silkworms fed by crop leaves were designated to produce partial animal nutrition for the crew. Various physical-chemical and biological methods were combined to reclaim wastewater and solid waste. Condensate collected from atmosphere was recycled into potable water through granular activated carbon adsorption, iodine sterilization, and trace element supplementation. All grey water was also purified though multifiltration and ultraviolet sterilization. Plant residue, human excrement, silkworm feces, etc. were decomposed into inorganic substances which were finally absorbed by higher plants. Some meat, ingredients, as well as nitrogen fertilizer were prestored and resupplied periodically. Meanwhile, the same amount and chemical composition of organic waste was dumped to maintain the steady state of the system. A nutritional balanced diet was developed by means of the linear programming method. It could provide 2721 kcal of energy, 375.5 g of carbohydrate, 99.47 g of protein, and 91.19 g of fat per capita per day. Silkworm powder covered 12.54% of total animal protein intakes. The balance of material flows between compartments was described by the system of stoichiometric equations. Basic life support requirements for crews including oxygen, food, potable and hygiene water summed up to 29.68 kg per capita per day. The coefficient of system material closure reached 99.40%. Ó 2009 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Bioregenerative life support system; Design; Nutrition; Stoichiometric modeling; Silkworm

1. Introduction Great developments of manned space flight have been flourishing in China since the end of the twentieth century. It plans to put the taikonauts on the moon and set up a permanent lunar base in years or decades. To support these manned space explorations, a series of Bioregenerative Life Support System (BLSS) studies has been conducting in Beihang University. It is generally accepted that physical-chemical (PC) approaches are cost-effective for short missions; while for *

Corresponding author. Tel./fax: +86 10 8233 9837. E-mail address: [email protected] (H. Liu).

the missions of extended duration and multiple-crew size, life support should be complemented by bioregenerative processes, although they are mostly untested and, in some cases, controversial (e.g. Jones, 2006). It has been demonstrated that higher plants could supply clean air, drinkable water, most food and good psychological state for maintaining a habitable environment (Perchonok and Bourland, 2002; Salisbury et al., 1997; Wolverton, 1980). Moreover, various effective waste processing techniques could also be employed to involve most material into biological turnover (Bubenheim and Wydeven, 1994; Gros et al., 2003; Wignarajah et al., 2000; Zolotukhin et al., 2005). Diets in the BLSS might well be largely vegetarian foods. But not every astronaut is vegan. Animals may also

0273-1177/$36.00 Ó 2009 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2009.11.022

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play an important part in food production system. Of course, some meat might be resupplied from the earth to the moon, even to the mars. Nevertheless, it might result in tremendous costs associated with current food preservation technology and space launch capacities (Cenci-McGrody and Stiller, 1997; Cle´ment, 2005; Lane et al., 2007). This article describes a conceptual scheme of a BLSS designed on the basis of the preliminary works (Liu et al., 2008; Xu and Liu, 2008; Yang et al., 2009; Yu et al., 2008). Silkworms as well as 15 kinds of higher plants were selected as food source in the BLSS. The purpose of this on-going study was to identify the mass flow characteristics of the BLSS for human missions on the permanent lunar or planetary base on a per capita per day basis. 2. Nutritional requirements One of the primary issues is to identify the adequate nutrient intakes for manned space flight. Nutrition concerns, including energy intake, macronutrient balance, vitamin and mineral deficiencies or excesses, and environmental factors, are especially important for space travelers, because they are exposed to a limited and closed environment for such a long time. The nutritional requirements depend on body mass, age, sex, and physical activity level. It is assumed that the BLSS inhabitants are healthy normotensive men, 30–60 years old, average body mass of 65 kg; and their physical activity levels are moderate during the mission. According to these assumptions, the recommendations of World Health Organization (WHO), National Aeronautics and Space Administration (NASA), and Chinese Nutrition Society (CNS) are summarized (see Table 1). However, nutrient requirements for long-duration space missions have not been accurately determined. Available literatures have suggested that the conditions of spaceflight do affect the levels of some nutrients required in the human diet. For example, the requirements of protein, calcium and iron have to be modified since they may cause healthy problems in space (Lupton and Turner, 2002; Smith and Zwart, 2008; Zerwekh, 2002). 3. System general description Basically, the term bioregenerative life support comprises four main functions: atmosphere revitalization, food production, waste reclamation and water recycling. Fig. 1 schematically depicts the essential components of the BLSS – higher plants cultivation, animal rearing, human habitation, water recovery, waste treatment, atmosphere management, and storages. 3.1. Higher plants cultivation One of the most important considerations in developing a sustainable BLSS involves selection of appropriate candidate crop species. Fifteen species of higher plants including

cereals, legumes, vegetables, spices and condiments were selected in accordance with a set of criteria to provide a complete nutrition and dietary variety (see Table 2) (Xu and Liu, 2008; Yang et al., 2002, 2004). Plants are grown in artificial substrate with hydroponic  solution. A mixture of about 3=7NHþ 4 =NO3 , which is commonly believed to be able to increase yield compared to NO 3 only, is used as nitrogen source (Gentry et al., 1989; Heberer and Below, 1989; Muhlestein et al., 1999).  Another benefit of using a mixture of NHþ 4 =NO3 is to maintain pH homeostasis in hydroponic solution (LeaCox et al., 1999; Mackowiak et al., 1990; Muhlestein et al., 1999). The artificial lighting, for example metal halide or high-pressure sodium lamps, are used to produce the mean photosynthetically active radiation (PAR) of 700–900 lmol m2 s1. The photoperiod regime depends on the plant species. For long-day plant, such as wheat and radish, it is 24 h constant light; and for short-day plant, such as rice and soybean, it is 12 h light/12 h dark. Atmosphere temperature in the plant growth chamber is maintained at 24–27 °C. An intensive agriculture will be practiced to grow a maximum quantity of usable biomass in an area as small as possible. For uniform and sustained oxygen production, the “conveyor-type” cultivation mode is employed. That is crops of different ages are grown simultaneously. 3.2. Animal rearing As a method to reduce the total mass of consumables to be carried, animal rearing could increase the degree of selfsufficiency of the BLSS. However, little attention has been focused in the past decades because of some certain problems connected with husbandry and slaughter of animals in the BLSS (Blu¨m et al., 1995; Salisbury and Clark, 1996). Animals such as sheep, cattle and chickens might eat the biomass that humans cannot consume. But they need large volumes, and particularly, would generate a great amount of odor and wastes (Yang et al., 2009). Compared with livestock and poultry, insects might become a good choice since they have played an important part in the history of human nutrition in Asia, Africa and Latin America (DeFoliart, 1992; Kok, 1983; Verkerk et al., 2007). Silkworms are traditionally acceptable as edible insects in China, Japan, Korea, Thailand, etc. They are easy to rear, have relatively short lifespan, require small space and few water, produce little odor and wastes (Katayama et al., 2008; Yang et al., 2009; Yu et al., 2008). Monotrophic silkworm possesses taste sensory organ, and recognizes mulberry leaves through its taste. Whereas, polytrophic silkworm, with its impotent taste sensor, may eat other plant leaves at the loss of its preference. Yu et al. (2008) confirmed that polytrophic silkworm (Bombyx mori) larvae could be raised with mixed leaves of mulberry and stem lettuce. The larval phase of silkworm, from hatch to the 3rd day of fifth instar, lasts about 25 days. They were fed on

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Table 1 Nutritional requirements. Nutrient Energy Protein Carbohydrate Fat Water Vitamin A Vitamin D Vitamin E Vitamin K Vitamin C Vitamin B12 Vitamin B6 Thiamin Riboflavin Folate Niacin Biotin Pantothenic acid Calcium Phosphorus Magnesium Sodium Potassium Iron Copper Manganese Fluoride Zinc Selenium Iodine Histidine Isoleucine Leucine Lysine Sulfur amino acids Aromatic amino acids Threonine Tryptophan Valine

Unit 1

kcal d %Total energy consumed %Total energy consumed %Total energy consumed mL d1 lgRE d1 lg d1 mg a-TEd-1 lg d1 mg d1 lg d1 mg d1 mg d1 mg d1 lg d1 mg NE d1 lg d1 mg d1 mg d1 mg d1 mg d1 mg d1 mg d1 mgd-1 mg d1 mg d1 mg d1 mg d1 lg d1 lg d1 mg d1 mg d1 mg d1 mg d1 mg d1 mg d1 mg d1 mg d1 mg d1

WHOa

NASAb

CNSc

This study

2881 7.5d 55–75 15–35 2900 600 5 15e 65 45 2.4 1.3 1.2 1.3 400 16 30 5 1000 700f 260 2500f 3100f 13.7g 0.9f 2.2f 4f 7h 34 150 650 1300 2535 1950 975 1625 975 260 1690

2875 12–15 50–55 30–35 >2000 1000 10 20 80 100 2.0 2.0 1.5 2.0 400 20 100 5 1000–1200 <1.5 times Ca 350 1500–3500 3500 10 1.5–3.0 2.0–5.0 4.0 15 70 150 ND ND ND ND ND ND ND ND ND

2737 11 55–65 20–30 2500 800 5 14 120 100 2.4 1.2 1.4 1.4 400 14 30 5 800 700 350 2200 2000 15 23 3.5 1.5 15.5 55 150 ND ND ND ND ND ND ND ND ND

2721 15 55 30 2672 942 NDi 23 ND 109 ND 1.2 1.4 2.6 416 21 ND ND 818 1396 522 2160 2866 12 2.8 3.8 ND 20 67 150 2184 3731 6974 4843 3416 7622 3405 892 4284

a

Abbreviation of World Health Organization. The data in this column were gathered from FAO/WHO (1995, 1998a,b), FAO/WHO/UNU (2001), Howard and Bartram (2003), WHO (2005), WHO/FAO (2003), and WHO/FAO/UNU (2007). Individual energy requirement is calculated with the WHO equation, accounting for proposed weight, age, sex, and moderate activity levels. The assumed body weight is 65 kg for all nutrients except iron. b Abbreviation of national aeronautics and space administration. The data in this column were gathered from Bourland et al. (2000), Lane and Feeback (2002), Lockheed Martin Space Operations (2002), and Wade (1989). The assumed body weight is 70 kg. c Abbreviation of Chinese Nutrition Society. The data in this column were gathered from Chinese Nutrition Society (2000). The assumed body weight is 65 kg. Individual energy requirement is about 95% of WHO value according to Chinese statistical results. d The protein requirements of adult men weighing 65 kg is 54 g d1 (0.83 g kg1 d1). Energy value of protein is 4 kcal g1 (WHO/FAO/UNU, 2007). Thus, the energy ratio of protein covered approximately 7.5%. e Data are not sufficient to formulate recommendations for vitamin E intake for different age groups except for infancy (FAO/WHO, 1998b). This data was extracted from Institute of Medicine, Food and Nutrition Board (2000). f Source: WHO (2005). These data were derived from Institute of Medicine, Food and Nutrition Board (1997, 2002, 2004), and Commission of the European Communities (1993). g Dietary iron bioavailability of 10%. Mean body weight of 75 kg (FAO/WHO, 1998b). h Dietary zinc bioavailability of 30% (FAO/WHO, 1998b). i ND, No data.

mulberry leaves during the first 15 days, and on 60% of inedible stem lettuce leaves as well as 40% mulberry leaves during the last 10 days. The yield and the growth rate of silkworm larvae reared using this method were a little lower than that feeding on mulberry leaves purely. Whereas, the protein content of mixed feeding larvae was

higher. No significant difference in amino acids contents was detected. Leaves of stem lettuce are usually regarded as inedible part for most people in China. Feeding silkworm with them could reduce the dependence on the mulberry leaves. Consequently, the mixed rearing protocol is employed.

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Fig. 1. Functional block diagram of the BLSS. T&H – temperature and humidity; GAC – granular activated carbon; UV – ultraviolet.

To decrease the age dependence of silkworm growth, silkworm larvae (Bombyx mori) are reared in a multistage manner. They are hatched in an artificial climate chamber under certain temperature and humidity. When they grow up to the early days of fifth instar (the nutrition derived from leaves has mainly contributed to the organization by this time), they could be harvest and cooked (Gu et al., 2002; Zou et al., 2001). 3.3. Water recovery The purpose of the water recovery subsystem was to supply potable and hygiene water, reclaim wastewater and monitor water quality. In the BLSS, condensate collected from the temperature and humidity control systems pass through granular activated carbon (GAC) adsorption, iodine sterilization, and trace element supplementation. The product water is expected to be of sufficient quantity and quality for potable. It is assumed that condensate can be collected entirely. The BLSS will process all water streams that are available regardless of the requirement of potable and hygiene. This might lead to an excess of potable water because of the large amount of condensate. Some of the excess water would be used for hygiene, while most of it would be recycled for irrigation.

Waste hygiene water is processed through multifiltration units. Water recovered from this processing is mixed with some condensate to make hygiene water via ultraviolet (UV) sterilization. It is assumed that the quality of product water from UV sterilization meets the hygiene water purity requirements. Toilet water, kitchen wastewater and water concentrated from multifiltration units have relatively high organic matter concentrations. These streams require a more rigorous treatment along with the solid wastes through biological and physical-chemical processes. Having been removed potentially toxic matter and added essential minerals, the effluent is benign to the plants growing from the hydroponic solution to which it is returned. Higher plants would complete the water recycling via transpiration. 3.4. Waste treatment Crop residue, which is a large portion of solid wastes generated from a BLSS, has relatively high concentrations of refractory wastes, i.e. cellulose, hemicellulose and lignin, and may also contain potentially harmful microbes (Bubenheim et al., 1997; Wydeven, 1988). The effective processing of solid wastes, particularly plant residue, is crucial to a successful BLSS. There are many earth-based technologies which are capable of processing the solid wastes, such as physi-

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Table 2 Basic data of crops, silkworms and stored fooda. Crop

Wheat Rice Soybean Peanut Pepper Carrot Tomato Coriander Cabbage Stem lettuce Radish Pumpkin Green onion Garlic bulb Mulberry leaves Silkworm Sugar Lard (rendered) Beef Luncheon fish Iodized salt

Energy (kcal)

368.9 350.7 420.8 584.7 31.5 41.0 21.4 35.6 26.9 16.1 24.5 24.9 35.5 130.2 97.7 62.8 399.6 897.2 229.1 382.6 0

Protein (g)

15.7 6.9 35.0 24.8 1.4 1.0 0.9 1.8 1.8 1.0 0.9 0.7 1.7 4.5 4.8 10.3 0 0 33.2 24.4 0

Fat (g)

2.5 0.7 16.0 44.3 0.3 0.2 0.2 0.4 0.5 0.1 0.1 0.1 0.3 0.2 1.2 2.0 0 99.6 10.7 30.2 0

Carbohydrate (g)

70.9 79.2 34.2 21.7 5.8 8.8 4.0 6.2 3.8 2.8 5.0 5.3 6.5 27.6 16.9 0.8 99.9 0.2 0 3.3 0

Water (g)

9.9 12.9 10.2 6.9 91.9 89.2 94.4 90.5 92.9 95.5 93.4 93.5 91.0 66.6 76.0 85.3 0 0.2 54 37.5 0.1

Ash (g)

1.0 0.3 4.6 2.3 0.6 0.8 0.5 1.1 1.0 0.6 0.6 0.4 0.5 1.1 1.1 1.6 0.1 0 2.1 4.6 99.9

HI (%)

40 30 38 40 35 50 46 93 93 60 50 54 90 53 31e 100 100 100 100 100 100

Yield rateb (g FWc m2 d1)

22.73 12.45 24.80 7.53 148.94 74.83 173.76 38.74 75.78 100.85 190.00 42.50 81.82 23.62 25.00

Daily required

Area required (m2)

(g FW d1)

(g DWd d1)

150.00 250.00 30.00 30.00 90.00 50.00 100.00 50.00 100.00 100.00 50.00 65.00 5.00 10.00 65.50 40.00 20.00 45.00 50.00 50.00 2.00

135.15 217.75 26.94 27.93 7.29 5.40 5.60 4.75 7.10 4.50 3.30 4.23 0.45 3.34 15.69 5.89 20.00 44.91 23.00 31.25 2.00

6.60 20.08 1.21 3.98 0.60 0.67 0.58 1.29 1.32 0.99 0.26 1.53 0.06 0.42 2.62

a

Content of major nutrients in 100 g of edible biomass were gathered from Yang et al. (2002, 2004). HI, harvest index. The yield rates (fresh weight) of edible crops were adapted from Adekpe et al. (2007), Berkovich et al. (2004), Dhaulakhandi et al. (1995), Hanford (2004), Nelson et al. (2008), Silverstone et al. (2003), Tako et al. (2008), and Wheeler et al. (2003). The yield rate of mulberry leaves was referred from Zhong (1991). c FW, fresh weight. d DW, dry weight. E Cited from Ke (1997). b

cal-chemical methods, biological methods, as well as their combination (Bubenheim and Wydeven, 1994; Finger and Alazraki, 1995; Kohlmann et al., 1993; Wydeven, 1988). Bioreactors are used as the primary treatment step for plant inedible biomass and other combined waste slurry including urine solid, fecal material, kitchen wastes, silkworm excrements, etc. These bioreactors depend on microbial species to oxidize organic carbonaceous and nitrogenous materials in the solid wastes. Biological waste processing systems can produce either nitrate or ammonium ions as a final product which can be reused directly by higher plants, but have long retention time and do not typically result in complete recovery of recalcitrant solid wastes, such as cellulose, hemicellulose, and lignin (Schlager, 1998; Strayer et al., 2002). Wet oxidation, which has been shown to be an excellent candidate technology for processing of wastes and recovering of inorganics (Tikhomirov et al., 2003; Wignarajah and Bubenheim, 1997), is proposed as a supplementary treatment step. Unlike incineration, wet oxidation does not require a predried feedstock. Either dilute or concentrated waste slurry can be oxidized using this technique. It is particularly attractive for BLSS since CO2 from combustion of organics and the cation in soluble form are readily available as plant nutrients. The product of wet oxidation may require further treatment, for example nitrification, before they

are reused because of incomplete combustion. The solid dissolved or suspended in resulting streams provide a major source for inorganic nutrient inputs to crop production. 3.5. Atmosphere management Atmosphere management deal with all the respiratory input and output needs of the crew, silkworms, as well as higher plants, which includes the control of gas composition, temperature, humidity, pressure, etc. (Klaus et al., 2005). Various interfaces exist with other life support subsystems, such as waste treatment. Carbon dioxide generated by human is combined with the carbon dioxide recovered from solid waste processing and delivered to the crops. Plants use the energy of light, absorb the carbon dioxide from the air, plus water and materials from the substrate, produce edible and inedible biomass, and emit oxygen. The photosynthesis by green plants completes the air revitalization step in the BLSS (Galston, 1992). Gas exchange between each component is assumed to be continuous. In this cyclic manner, autotrophs and heterotrophs are mutually dependent upon each other. To maintain high reliability of the BLSS over long periods of time in a hostile environment, the buffers of oxygen and carbon dioxide are extremely necessary.

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The details of atmosphere management subsystem, including gas separator, buffers, sensors and control systems etc., are not depicted in Fig. 1.

17:7109 CO2 þ 14:1683 H2 O þ 0:2261 NH3 þ 0:5275 HNO3 ! 0:0301 C100 H159 O31 N25 protein

3.6. Storages Up to now a complete closure has never been achieved by means of an artificial ecosystem. Storages of food and minerals resupplied from earth (input), as well as organic matter to be removed (output), have always been necessary. Enough seeds and silkworm eggs for generation alternations were prestored with other infrastructure logistics before system was sealed. Mass of these materials would not be involved into following calculation process. Five main food items: Beef, Lard (rendered), Luncheon fish, Iodized salt, Sugar, were prestored and resupplied periodically. They are all precooked and individually packaged so they are either ready to eat or can be prepared simply. To maintain a sustainable system, some organic waste should be dumped overboard or returned to earth. Storing most kinds of solid waste will require some pretreatment to minimize the production of noxious gases and odors. 4. Stoichiometric modeling As indicated in Fig. 1, this BLSS design incorporates synthesis, consumption, and degradation of organic matters. For calculating mass exchange, approaches similar to Volk and Rummel (1987) were used to simulate the steady state of each compartment and consequently the whole system. Each compartment was described by one or more stoichiometric equations made up of a set of constituents. The coefficients of equations were determined in accordance with some constraints. For example, the number of silkworms was determined by the amount of lettuce since all lettuce leaves were used for silkworm rearing in the rules discussed in Section 3.2. In determining the stoichiometry of biomass synthesis, the plant consumption of both ammonia and nitrate nitrogen was taken into account with a portion of 3/7. Eqs. (1)–(8) also provides the elemental composition of the constituents on the basis of four main elements C, H, O, and N (the elements S and P will be added in the future). Elements in sewage sludge (He et al., 1998) gave reference to estimate the composition of intermediate product of waste treatment. Other constituents’ formulas were taken from (Manukovsky et al., 2005). Before the actual BLSS is set up and operated, stoichiometric model (Eqs. (1)–(8)) could be employed to carry out a preliminary analysis of the complete loop for the equilibrium condition. Process kinetics, such as rates of biological reactions described by stoichiometries, was ignored in the static analysis. Plant growth: Edible biomass generation

þ 0:0284 C57 H104 O6 þ2:1800 C6 H10 O5 þ19:5840 O2 fat

carbohydrate

ð1Þ Inedible biomass generation 30:8612 CO2 þ 23:8594 H2 O þ 0:1189 NH3 þ 0:2774 HNO3 ! 32:647 O2 þ 0:0158 C100 H159 O31 N25 protein

þ 0:0215 C57 H104 O6 þ0:4058 C6 H10 O5 þ3:2488 C6 H10 O5 fat

carbohydrate

cellulose

þ 0:1856 C33 H38 O12

ð2Þ

lignin

Human metabolism: 0:0451 C100 H159 O31 N25 þ0:1030 C57 H104 O6 fat

protein

þ 2:3160 C6 H10 O5 þ25:3628 O2 ! 22:3852CO2 carbohydrate

þ 18:3266 H2 O þ 0:0067 C100 H331 O86 N151 urine

þ 0:0076 C100 H170 O61 N5 þ0:0046 C100 H190 O40 N16 feces

ð3Þ

body waste

Silkworm growth:   0:0030 C100 H159 O31 N25 þ0:0015 C57 H104 O6 þ0:0860 C6 H10 O5 þ0:0386 C6 H10 O5 fat

protein

carbohydrate

cellulose

þ 0:4030 O2 ! 0:4013 CO2 þ 0:3347 H2 O   þ 0:0019 C100 H159 O31 N25 þ0:0009 C57 H104 O6 þ0:0020 C6 H10 O5 fat carbohydrate silkworm protein  þ 0:0011 C100 H159 O31 N25 þ0:0005 C57 H104 O6 protein fat  þ 0:0366 C6 H10 O5 þ0:0198 C6 H10 O5 carbohydrate

cellulose

leaves

ð4Þ

worm excrements

Waste treatment: Biological process 18:4807 O2 þ 0:0137 C100 H159 O31 N25 þ0:0199 C57 H104 O6 fat

protein

þ 0:3530 C6 H10 O5 þ3:1466 C6 H10 O5 þ0:1788 C33 H38 O12 carbohydrate

cellulose

lignin

þ 0:0067 C100 H331 O86 N151 þ0:0076 C100 H170 O61 N5 feces

urine

þ 0:0046 C100 H190 O40 N16 ! 20:4049 CO2 þ 13:1004 H2 O body waste

þ 0:1089 C100 H209 O41 N9 þ0:4899 NH3

ð5Þ

solid residue

Wet oxidation 6:7261 O2 þ 0:0538 C100 H209 O41 N9 solid residue

! 5:3809 CO2 þ 4:8966 H2 O þ 0:4843 NH3

ð6Þ

Nitrification 1:2584 O2 þ 0:6292 NH3 ! 0:6292 HNO3 þ 0:6292 H2 O

ð7Þ

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Input–output balance: 0:0130 C100 H159 O31 N25 þ0:0737 C57 H104 O6 þ0:1340 C6 H10 O5 protein

fat

carbohydrate

þ 0:7403 H2 O þ 0:1757 HNO3 ! 0:0003 C100 H159 O31 N25 protein

þ 0:0006 C57 H104 O6 þ0:0035 C6 H10 O5 þ0:0834 C6 H10 O5 fat

carbohydrate

þ 0:0067 C33 H38 O12 þ0:0550 C100 H209 O41 N9 lignin

cellulose

ð8Þ

solid residue

5. Results and discussion The general scheme of the mass exchange estimated by the Eqs. (1)–(8) is illustrated in Fig. 2, where arrows represent the fluxes and objects represent the subsystems. Solid, liquid, and gas flows are all involved to predict the behavior of the BLSS. 5.1. Food provision Food preparation takes the edible biomass of crops as well as silkworms, and produces food products which would be consumed together with meat and ingredients supplied from the Earth. A linear algebraic model was developed to provide a nutritionally balanced diet to the crew. Table 2 lists one of the optimal combinations of dietary components. In this recipe, daily fresh food consumption is 1287 g d1 (water content of 54.87%). Most of the

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food, 83.92% in fresh weight (78.12% in dry weight), is supplied by crops. The ratio of animal protein to total protein consumed is 33.10%. And silkworm covers 12.54% of total animal protein intakes. This recipe could provide 2721 kcal, 375.5 g of carbohydrate, 99.47 g of protein, and 91.19 g of fat per capita per day. However, these values may vary depending on the activity level at which he is operating. Nutrient contents for vitamin D, vitamin B12, vitamin K, biotin, pantothenic acid and fluoride of some food species were not available. Despite this, the menu proposed could almost meet all the other requirements of indispensable amino acids, minerals and vitamins. Note that vitamin D plays a very important role in the calcium absorption from the intestine, as well as other calcium-related effects in kidney and bone. People in the space station may have a deficiency in vitamin D because of the inadequate sunlight exposure. A supplement of vitamin D would be necessary (Wade, 1989). There should be a balance of different foods not only on every day, but also even more important, over the week and the month. The data presented in Table 2 are mean values of dietary intakes over a few days. 5.2. Human requirements Crew members in BLSS continuously exchange matter and energy with the surroundings. Table 3 shows the aver-

Fig. 2. System mass flow rates. All values are given in g capita1 d1.

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Table 3 Average values for human inputs and outputs in BLSS. Unit: g capita1 d1. Inputs

Mass

Output

Mass

Oxygen Vegetable food (dry weight) Water in vegetables Silkworm Food from storage (dry weight) Water in storage food Potable water Oral hygiene water Hand/face wash water Urinal flush water Laundry water Shower water (once per two days) Dishwashing water

811.6 453.7 626.3 40.00 121.2 45.84 2000 370.0b 4080b 500.0b 12,470b 2720b 5440b

Carbon dioxide Organic solids of urine Urine water Organic solids of feces Feces water Miscellaneous body wastea Respired & perspired water Waste oral hygiene water Waste hand/face wash water Waste urinal flush water Waste laundry water Waste shower water Waste dishwashing water Ash Total waste

985.2 33.80 1500 18.33 150.0 10.31 1387 370.0 4080 500.0 12,470 2720 5440 14.60 29,679

Total consumables a b

29,679

These include sweat solids, dead skin cells and associated oils, hair, saliva solids, mucus, finger and toe nails, etc. Referred from Hanford (2004).

aged estimates of the daily inputs and outputs needed for sustaining humans. The number of basic life support requirements sums up to about 29.68 kg capita1 d1, about 95.19% of which is water. Potable water supplied is 2 l per day. Water required for person hygiene purposes, laundry and toilet flush sums up to 25.58 kg capita1 d1 (Hanford, 2004). Using the numbers in Table 3 we can see that in 1 year a 65 kg adult man requires 4.56 times his weight in oxygen, 7.23 times his weight in food (fresh weight), and 154.9 times his weight in water. And that is without packaging. Moreover, it would be continue to increase linearly as mission duration and crew size increase (Bartsev et al., 1996a,b). For exploratory missions, it becomes impractical to rely on stored supplies since the trip from the earth to the moon or the mars is so expensive. To permit longer manned missions, such as permanent lunar base, ultimate closed-loop BLSS have to be introduced (Horneck et al., 2003; Tamponnet, 1996).

5.3. Plant growth The area occupied by each higher plant is calculated as: S = M/Y, where S is the planting area (m2); M is the daily requirement of edible biomass or leaf mass (g fresh weight d1); Y is the nominal yield rate (g fresh weight m2 d1) (see Table 2). Planting densities depend on the plant species and available lighting. The total growth area required is approximately 42.21 m2 per capita. Rice covers nearly half of total area (47.57%), followed by wheat (15.64%) and peanut (9.43%). The area required for Mulberry tree is about 2.62 m2, which is 6.21% of the total area. The production rate of inedible plant biomass is about 1641 g capita1 d1, including 132.1 g of silkworm feed, i.e. the leaves of stem lettuce and mulberry tree. Rice straw accounts for about 35.55% of all inedible biomass in the sys-

tem. Comparatively, wheat straw occupies 13.71% because of the lower diet requirement and higher edible yield rate. The crop transpiration rate depends on the comprehensive function of many factors, such as plant species, productivity, atmosphere temperature, relative humidity, photoperiod regime, PAR, and CO2 concentration (Jacquez, 1990). In the CEEF, the overall transpiration rates varied from 4.4 to 6.3 L m2 d1 (Tako et al., 2007, 2008), while 1.8–4.7 L m2 d1 in the NASA’s Biomass Production Chamber (Wheeler et al., 2008). Mackowiak et al. (1990) estimated that transpiration rate in a BLSS is at least 4.5 L m2 d1. This value is employed in this study. The production of water via transpiration is approximately 190 kg capita1 d1. It is about 68.8 times greater than the rate of biomass production. 5.4. Silkworm rearing Daily yields of stem lettuce leaves and mulberry leaves are 66.67 g and 65.47 g, respectively. Four hundred and fifty-six silkworms are thought to be capable of living on this amount of leaves based on the mixed rearing protocol described in Section 3.2. Eq. (4) is generally based on the experimental results. By solving this equation, an average of 13.67 g d1 (dry weight) excrement is generated in the BLSS in which silkworm provide 40 g d1 (fresh weight) edible biomass for diet. To gain 1 kg of edible biomass, cattle and sheep need more than 10 kg of feed, swine need about 5.6 kg, broiler chicken apparently need 3.1 kg (Phillips et al., 1978; Schwartzkopf, 1992), and silkworms in this study require about 3.3 kg of leaves. The production efficiency (i.e. the gain in edible biomass divided by the mass of feed) of silkworm is slightly smaller than broiler chicken, while it is much greater than the live stock. Moreover, the edible proportion of silkworm (100%) is much higher than chicken (about 60%) (Phillips et al., 1978; Schwartzkopf, 1992).

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At this point, silkworms may have a competitive advantage in space diet. 5.5. Waste streams The organic wastes in the BLSS come mainly from the higher plants, which is about 90.77% in dry weight. It is approximately 10.6 times greater than human body waste (dry weight). Silkworm excrement only account for 1.39% of total solid waste (dry weight). Estimates of human liquid waste (i.e. urine water, feces water, respiration and perspiration water) is on an average 3.0 L capita1 d1, while hygiene liquid waste is assumed to be equal to the provision (i.e. 25.58 L capita1 d1). Combined, the mass of liquid waste is approximately 15 times greater than inedible biomass (fresh weight). Water purification is particularly relevant in a BLSS since liquid waste is the dominant waste streams (Garland et al., 1997). 5.6. Coefficient of system closure The coefficient of closure (R) is defined as the relation of mass supplied into the system from outside per time unit (m) to the mass of all substances consumed by the crew during the same time (M) (Bartsev, 2003). m ð9Þ R¼1 M As can be seen in Fig. 2, 167.0 g capita1 d1 of food and 11.07 g capita1 d1 of nitrate are imported, while the daily total consumption is 29,679 g capita1 d1. The coefficient of material closure reaches 99.40%, which is higher than the system without silkworms (98.68%) (Liu et al., 2008). In this study, solid waste does not need to be decomposed completely since system closure is not 100%. Waste treatment subsystem must store some wastes which would return to earth or dump into space, while provide the ability to transfer certain mass to relevant subsystems for material recycling. To save power penalty, it is better to dump the raw waste material and/or intermediate waste product. As revealed by the Sections 5.3 and 5.5, rice straw is the main portion of the solid wastes. Thus, it would be the first choice to be dumped. Eq. (8) illustrates an example of balance between inputs and outputs of the system. The composition of rice straw was analyzed through an experiment. To maintain the total amount of substances in the system, 27 g of rice straw and 151 g of solid residue (i.e. the intermediate product of waste processing) are removed. 6. Conclusion China would establish a terrestrial model of the BLSS in a few years. In order to achieve an active role in the future space operations, a design scheme conceptualizing the fluxes and stocks of the system is proposed. Higher plants are employed as the main contributor to provide a buffer

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