Composting And Food Waste Biology Essay

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Introduction

Food waste has been a big problem all over the world. According to Hong Kong Environmental Protection Department(HKEPD, 2008), about 9,300 tonnes of municipal solid waste (MSW) was disposed at landfills everyday. The organic fraction of the MSW was mainly food waste and yard waste, which amounts to over 3,600 tonnes, constituting about 38% and is the largest fraction of the MSW. The disposal of such biodegradable waste direct to the landfills causes a problem in Hong Kong as it leads to rapid depletion of the limited landfill void space and the formation of landfill gas and leachate that impose long term environmental burden on our environment.

Hong Kong government has started addressing the problem of food waste by proposing some biological treatments such as composting and anaerobic digestion since 2005. Food waste undergoes a natural decomposition by the microbes during composting. It turns the organic substrate into a stabilized organic form which can acts as nutrients to the plants.

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phosphate ( MAP , MgNH4PO4 .6H2O) as described in the following equation:

Mg2+ + PO4 + NH4+ + 6H2O = MgNH4PO4.6H2O (Nelson et al, 2003)

The formation of struvite helps to retain a high fraction of nitrogen content in the compost and form a high value fertilizer as it is a slow releasing fertilizer providing N, P and K to the plants. (Ren et al.,2010) had found that the molar ratio of Mg to PO4 equals to 1 to 2 was suitable for the struvite formation; therefore Mg to PO4 as 0.05 M to 0.1 M was used in this experiment. Moreover, 0.05M MgO was used since some research show that the concentration of MgO higher than 0.05M inhibited organic matter decomposition during composting. (Lee et al,2009) Further, to maintain the typical struvite formation ration, a treatment containing Mg to PO4 as 0.05 M to 0.05M was also used for comparison.

formation is 8-9 (Lee et al., 2009). Therefore, addition of lime was considered to alleviate the low pH condition as well as to promote the struvite formation

Research Objective and Outlines

The objective of this experiment is to evaluate the effect of lime, MgO and K2HPO4 addition in different concentrations in food waste composting by comparing(i) the decomposition performance, (ii) the nutrient value, (ii) the struvite formation and (iii) maturity time of compost.

Chapter 2 Literature Review

2.1 Definition of composting

Composting is a natural decomposition process that the organic substrate degraded by the microorganism into a stabilized organic substance under an aerobic condition. (Haug, 1993) It is an environmental friendly and cost effective method to fix the waste problem. As the some phytotoxic substance can be degraded together with the organic matter and the matured compost can be used as fertilizer.( de Bertoldi et al, 1983)

However, the nutrient value in the compost will be greatly reduced from the loss of nitrogen in form of ammonia during the composting process. The ammonia emitted will also lead to a odor problem. It is very important to reduce the amount of ammonia emission and hence solving the serious odor problem and producing high quality fertilizer.

2.2 Factor affecting decomposition process

2.2.1 Microbial activity

There are many different microorganisms in the compost mass during the composting process. They are responsible for different part in the degradation of organic matter. Bacteria, fungi

and also actinomycetes take a important role during the composting.

Bacteria is the most crucial in degradation of compost. There are different species of bacteria dominate in different phrase during composting. For example, Staphylococci dominate in mesophilic growth phase (Hassen et al,2001); Bacillus and Clostridium dominate in thermophilic phase .(Strom, 1985b)

At the later stage, the temperature of the compost decrease which facilitate the growth of actinomycetes.( Miller, 1996) For example, the Streptomyces and Thermomonospora dominate in later stage of composting.

Fungi and also actinomycetes will use up the carbohydrate remained such as lignin, cellulose and also hemicelluloses.( Miller, 1991)

2.2.2 Nutrients (C/N ratio)

Nutrient influences the microbial growth. Carbon and nitrogen are the nutrients which are most important for the microorganism in the compost. Carbon supports the cellular growth of the microorganism while the nitrogen can support the protein synthesis of microorganism. The best C/N ratio for the growth of microbes is 30:1 ( Epstein, 1997) The composting process will be hindered by the wrong initial C/N ratio. If the C/N ratio is as low as 15, which spend much more time to be mature than that of the waste with C/N ratio 30. Besides slow degradation rate, the low C/N ratio will also promote the nitrogen volatilization to release ammonia gas which resulted in a low quality fertilizer. (Epstein, 1997) When C/N ratio is

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2.2.3 Moisture content

Water is essential for all microorganisms during composting as water can support their life and also mobility to gain food from the surface of the waste. (Rynk, 2000) The ideal moisture content for composting would be 50-60% ( Rynk, 2000) that which can be adjusted by adding water or sawdust. If the moisture content is higher than 60%, an anaerobic condition is resulted which suppress incoming oxygen (Seekins, 1999) by the water as the porosity of compost material is reduced with the high compaction . (Das and Keener, 1997) However, if the moisture content lower than 40%, the microbial activity will reduced as the microbial metabolism is hindered which resulted in a immature compost.

2.2.4 Temperature

Temperature affects the community and also activity of the microorganism and hence affecting the degradation of the compost. Sanitation can be achieved in the high temperature which can remove the pathogen in the compost and allow the compost be mature.(Epstein, 1997) However, the high temperature excced 80℃ ceases the microbial activity and greatly reduce the decomposition rate of the compost.

2.2.5 pH

The decomposition rate of compost is highly controlled by pH. When the pH is 6 or below, the initial decomposition is reduced(Nakasaki et al, 1993) as the alkaline condition favor the microbial activity and community. When under low pH, the volatile fatty acid is promoted (Brinto,1998)while ammonia gas is promoted under high pH owing to the ammonium volatilization. (Gage, 2003) However, the quality of compost is lowered by increasing loss of nitrogen.

2.3 Factor affecting nitrogen loss

During composting, the organic nitrogen transformation by the microorganism will produce ammonia as byproduct. The ammonia can be in form of volatile gas or retain in the compost. Presence of ammonia is important as it can increase the pH in compost in order to enhance the degradation of organic matter. (Jeong and Hwang, 2005) The retained ammonia will converted to a nitrate by nitrifying bacteria, especially at the maturing stage during the composting(less than 40℃)(Sanchez-Monedero et al., 2001) There are some factors affecting the ammonia emission rate. Firstly, the low C/N ratio will release more ammonia gas as there are less organic matter to support the growth of the microorganism which in turn lower the nitrogen assimilation by microbes and hence increasing the retention time of nitrogen remaining in compost. For example, the loss of nitrogen can up to 43-62% of original nitrogen, depending on the amendment added.(Eklind and Kirchman, 2000) Other study also shown that the loss of nitrogen in high nitrogen containing municipal solid waste can amount up to 40%.( Sanchez_ Mondedero et al, 2001) Secondly, the temperature will also increase the ammonia volatilization rate as ammonia will be more easy to be converted into gaseous form under high temperature. Thirdly the aeration rate also affects the nitrogen loss in gaseous form as the higher the aeration rate the more the ammonia gas carried away to air outside the composter. Besides, the mixing and turning of compost will also account for the higher ammonia volatilization rate.( Morisaki et al, 1989)

The nitrogen may also loss in form of nitrogen oxide in gaseous form which can amount up to 20% of initial nitrogen.(Bernal et al, 1993)

2.4 Struvite formation

2.4.1 Optimum formation condition

Struvite is a crystal called magnesium ammonium phosphate which formed by magnesium, ammonium and phosphate in a ratio of 1:1:1 in the following equation.

Mg2+ + PO4 + NH4+ + 6H2O = MgNH4PO4.6H2O (Nelson et al, 2003)

pH value, temperature, other kinds of ion, solubility of struvite and the degree of supersaturation.are crucial for the struvite precipitation.( James and Simon, 2002). Struvite can form from pH 7 to 11 while the optimum pH for struvite formation would be 8.5 to 9.2 in wastewater.(Uludag-Demirer et al, 2005)

2.4.2 Advantage of struvite formation in compost

As ammonium ions are captured by the magnesium and phosphate ions for the formation of struvite, the nitrogen content in the compost can be conserved and hence less odor problem resulted as less ammonia gas emitted. (Jeong and Kim, 2001)The maximum recovery of ammonium- nitrogen can reach 92 % in landfill leachate (Uludag-Demirer et al,2005) while it can reach up to 95% under aerobic digestion. (Ganrot et al, 2007) Struvite is slow releasing fertilizers which only slightly soluble in neutral or alkaline condition while it is readily soluble in acidic condition. (D.G.Chirmuley, 1994) Moreover, it is not easily flashed away by rain water. Therefore, it would be more effective when applying on sloppy and acidic area. (L.Pastor et al, 2008)

Chapter 3 Methodology

3.1 Composting control system

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There was a computer controlling five 20L stainless steel containers where the composting process carried out inside. There was a metal lid and a plastic "O" ring covering the composter which was screwed tightly by eight screw in order to prevent the leakage of gas from the composter.

A layer of heat insulating foam was injected surrounding the composter in order to reduce the heat loss during composting. The structure of composter were as shown in Figure 3.1.

Figure 3.1 Diagram of insulating structure of a composter

3.2 Preparation of food waste mix

Food waste were made from bread, vegetable, rice and boiled pork in ratio of 13:10:10:5 artificially. All the food waste was cut into 1 cm3 for homogenous mixing. The C/N ratio was adjusted by adding 9100 g oven-dried sawdust. For each tank, about 7.0 kg of the mix were added, and then the lime, Magnesium oxide and dipotassium hydrogen phosphate were added in different concentrations. 500 g plastic beans was added to each tank to achieve a bulk density of about 0.5kg/L. The moisture content were adjusted into a 55% by adding 1.5L dionized water. The aeration was set in 0.5 L min-1 kg-1 DW for the first week and then set to 0.25 L min-1 kg-1 DW afterwards.

3.3 Treatment and experimental design

There are five treatments in my experiment, R1 is the control without any addition. R2 with lime added only as lime provide a alkaline condition favoring the composting process. For R3 and R4, both with the addition of lime , magnesium oxide and dipotassium hydrogen phosphate. However, Magnesium ion to phosphate ion ratio was 1:1 in R3 while it is 1:2in R4. These two treatments were set to compare the better ratio for the struvite formation during composting. Besides, R4 and R5 had the same concentration of Magnesium and phosphate but there was no lime addition in R5 which is set to investigate whether lime favor the formation of struvite.

Table 3.1 Experimental designs of different treatments in food waste composting

Treatment

Component

Lime

(g per Tank)

MgO

(g per Tank)

K2HPO4

(g per Tank)

R1

Control(without addition of lime, MgO and K2HPO4)

0

0

0

R2

Addition of 2.25% lime,

70

0

0

R3

Addition of 2.25% lime, 0.05M MgO and 0.1M K2HPO4 (Mg: P = 1:2)

70

6.3

54.81

R4

Addition of 2.25% lime, 0.05M MgO and 0.05M K2HPO (Mg: P = 1:1)

70

6.3

27.4

R5

Addition of 0.05M MgO and 0.1M K2HPO (Mg: P = 1:2)

0

6.3

54.81

3.4 Gas injection and collection

The following Figure 3.2 shows the experimental setup of the composter. Air was flowed from the aeration pump and passed into the composter. The gas emitted was released into a fume hood in normal condition.

When measuring the ammonia gas emission, the efflux of gas was collected to a iced water condenser and then a conical flask containing the 100ml 0.4M boric acid to trap the ammonia for an hour.

When measuring carbon dioxide evolution, the outgoing air was collected to a moisture trap to remove the moisture in the gas and measure the carbon dioxide evolution by the carbon dioxide analyzer.

Figure 3.2 Diagram of experimental setup

3.5 Compost sampling

There are eight sampling days in my experiment, they were day 0, 3, 7, 14, 21, 28, 42 and 56. In each sampling day, the food waste from five composters were mixed in different tanks. The mixing days were day 0, 3, 7, 10, 14, 17, 21, 28, 35, 42 and 56. During mixing the compost, water was added by look and feel method to maintain the water moisture of compost to around 60%.(Rynk, 2000) 100g of duplicate samples were obtained from each composter.

3.6 Gas analysis

3.6.1 Carbon dioxide

The outgoing gas was collected to a moisture trap which composed of silica gel and then clonnected to a carbon dioxide analyzer(WMA-3,P P System, UK). The reading of carbon dioxide were obtained.

3.6.2 Ammonia

Ammonia was trapped by a conical flask containing 0.4M boric acid solution with bromocresel green- methyl red indicator. The ammonia trap was replaced every day. The solution obtained after connecting to the outgoing gas of composter was titrated against 0.1M HCl to give a blood red end point.

3.7 Compost quality analysis

There were 10 parameters carried out in the experiment to determine the compost quality of food waste. They were pH, EC, moisture content, extractable ammonium, total nitrogen, nitrate and nitrite, total phosphorus, total organic carbon, total organic matter and seed germination index. For preparing the water extract, 20g fresh compost sample was shaken with DI water at a ratio of 1:5 dry weight per volume for one hour and kept it still for half an hour. pH was measured by Orion 920A ISE ionanalyser and EC was measured by Orion 160 conductivity meter after the water extraction of compost. The suspension in the water extraction was removed by the centrifugation at 13500 rpm for 20 minutes before filtered out by passing through 0.45µm membrane filter. The filtrate was used for seed germination test, indophenols-blue method, Cadmium reduction method for the analysis of maturity of compost, extractable ammonium and also nitrate respectively.

The moisture content are measured by gravimetric method that the fresh compost was weighted and put into 105℃ oven for 48 hrs (Rynk, 2000) The oven dried samples were then being put into a 550℃ oven for 16 hours to measure the TOM. While TOC was determined by the Walkey- Black method by using air dried samples which were put in the 55℃ oven for 24 hrs. The air dried samples were used for indophenol blue and vandomolybdphosphoric acid method after acid digestion for total nitrogen and total phosphorus determination respectively.

3.8 Precipitate analysis

The dried samples were grinded and analyzed by X-ray diffraction through a X ray diffractrometer. (XRD, Bruker D8 Advance Xray Diffractrometer) The powder XRD pattern of treatments were compared with the XRD pattern of standard struvite by matching the position and also the intensity of peak of the crystal structure.

Chapter 4 Result and discussion

4.1 Change of temperature

Temperature affects the activity and population of microorganism which in turn affects the rate of decomposition. Besides, it achieves sanitation if the temperature remains higher than 55 ℃ to 60℃ for three day, it can kill the pathogen in the compost and turn it to be mature.( Stentiford, 1996)

In Figure 4.1, all the treatments raised to a high temperature at the beginning. However, R1(artificial food waste without any addition) and R4 (Addition of 2.25% lime, 0.05M MgO and 0.1M K2HPO4 ) could not maintain a high temperature for a period of time and dropped back to room temperature quickly which were caused by the acid inhibition and hence less heat released. For other treatments, both were able to maintain a high temperature for at least one week which indicated that they underwent a normal composting process.

Fiugure 4.1 Change of temperature during the food waste composting with different concentration of lime, Magnesium oxide and dipotassium hydrogen phosphate.

4.2 Change of pH

The initial pH varied because of different lime addition. There was a general decreasing trend in the first few days. There were two reasons for the decrease of pH. It might decrease because of the microbial converting the ammonium ion into microbial biomass aerobically.(Beck-Fiss et al,2003) Besides, it may also dropped due to the organic acid formed during the decomposition of organic matter. (Nakasaki et al,1993)

In Figure 4.2, the pH of control dropped at the beginning and kept at a low pH afterwards. While R4(Addition of 2.25% lime, 0.05M MgO and 0.1M K2HPO4 ) dropped into a low pH for a period and then raised to a higher pH later. While the other treatments maintained in a higher pH which favored the composting process which indicated the addition of lime can buffering the organic acid formation under a low pH. ( Nakasaki et al,1993)

Figure 4.2 Changes of pH during the food waste composting with different concentration of lime, Magnesium oxide and dipotassium hydrogen phosphate.

4.3 Changes of carbon content in compost

4.3.1 Carbon dioxide evolution

The carbon was mainly lost in from of carbon dioxide as carbon dioxide released during the decomposition of carbohydrates.(Baca et al, 1993) The carbon dioxide increase proportional to the microbial activity.

In Figure 4.3, all the treatments increase dramatically at the beginning and then decrease afterward. Control and R4 decrease quickly less than a week due to the acid inhibition. While the other treatments maintained a high CO2 evolution for a week which indicated a high microbial activity and hence a normal composting process. The evolution are gradually decrease due to the available nutrient are readily depleted for the microorganism.

From Figure 4.4, R2, R3, and R5 has a higher than double of cumulative CO2 evolution to that of control and R4. The low CO2 evolution in R1 and R4 are mainly caused by the low pH which inhibit the microbial activity and hence lower the decomposition of organic matter.

Figure 4.3 Daily carbon dioxide evolutions during the food waste composting with different concentration of lime, Magnesium oxide and dipotassium hydrogen phosphate.

Figure 4.4 Cumulative carbon dioxide evolution during the food waste composting with different concentration of lime, Magnesium oxide and dipotassium hydrogen phosphate.

4.3.2 Changes of total organic matter

The reduction of total organic matter is used to indicate the decomposition rate as the organic matter is degraded. (Epstein, 1997) The changes of total organic matter should be correlated with the CO2 evolution. From Figure 4.5, the R2, R3 and R5 had a large reduction of TOM, which also had a high CO2 evolution.

While the control and R4 had a smaller reduction of TOM which correlated with the low CO2 evolution. The slow reduction of CO2 evolution was caused by the acid inhibition on the microbial activity and hence less organic matter was decomposed.

Figure 4.5 Changes of total organic carbon during the food waste composting with different concentration of lime, Magnesium oxide and dipotassium hydrogen phosphate.

4.3.3 Changes of total organic carbon

Total organic carbon is also an indicator for the degradation rate. It also decreased against the CO2 evolution due to the mineralization if carbon in food wastes to the carbon dioxide.

As Figure 4.6 shows, TOC reduce with the composting days. Control and R4 nearly had no change on the TOC reduction due to the low pH hindering the microbial activity and resulted in a low degradation rate. After 56 days, 0.64%, 14.18%, 2.54%, 18.47% and 20.90% of total organic carbon were degraded in R1, R2, R3, R4 and R5 respectively. The degradation rate is the highest in R5 which also had the highest CO2 evolution. The TOC content in other treatments also correlated to the CO2 evolution.

Figure 4.6 Changes of total organic carbon during the food waste composting with different concentration of lime, Magnesium oxide and dipotassium hydrogen phosphate.

4.4 Changes of nitrogen content in compost

4.4.1 Changes of soluble ammonium

From Figure 4.7, It increased at the beginning in all treatment due to the ammonification carried out by the microorganism. There are three reasons account for the decrease of ammonium afterward. Firstly, it removed in form of ammonia. Secondly, it was taken up by the microorganism for growth which could be represented by equation 1 Thirdly, it converted to nitrate by microbial activity which could be represented by equation 2.

NH4+ + 2O2  NO3- + 2H+ + H2O---------------------------Equation 1(Polprasert,1989)

NH4+ + 4CO2 + HCO3- + H2O  C5H7O2N + 5O2-------Equation 2(Polprasert,1989)

R1 and R4 gave a very low amount of extractable ammonium as the microbial activity was inhibited by the low pH and hence hindered the ammonification which carried out by microorganism.

R2 and R3also had a high amount of extractable ammonium as lime is added which produced a alkaline condition to promote the growth of microorganism and also to facilitate the ammonification. (Jeong and Hwang, 2005)

While R5 produced less ammonium than R2 and R3 as there was no lime addition to provide a alkaline condition to enhance the ammonification. Besides, the formation of struvite captured the ammonium ion which also lower the amount of soluble ammonium in R5.

Figure 4.7 The changes of extractable ammonium during the food waste composting with different concentration of lime, Magnesium oxide and dipotassium hydrogen phosphate.

4.4.2 Ammonia emission

During the degradation of protein in the food waste, ammonia would be released under a high pH condition.(Nakasaki et al, 1993) Volatilization of nitrogen containing compound lead to the formation of ammonia gas under a alkaline condition.(Jeong and Hwang, 2005) Its mainly caused by the shift of equilibrium under high pH value by the action of microorganism and as shown in the following equation.

2 NH3 (l)  NH4 (aq) + NH2 (aq)----------------------Equation3

From Figure 4.8, there was a sharp increase of ammonia emission for first two weeks form all treatments except control and R4. Control and R4 only had a small amount of ammonia released as their degradation rate is too low to produce high concentration of ammonia and their pH value is too low to shift the equilibrium to emit ammonia gas.

While R2 had the highest emission because of its high pH value. For R3 and R5, the ammonia released is much less than that of treatment 2 as the there were precipitates formed which capturing the ammonium ions and hence reduce the ammonia loss.

Figure 4.8 Changes of daily ammonium gas emission during the food waste composting with different concentration of lime, Magnesium oxide and dipotassium hydrogen phosphate.

Figure 4.9 Changes of cumulative ammonium gas emission during the food waste composting with different concentration of lime, Magnesium oxide and dipotassium hydrogen phosphate.

4.4.3 Changes of total nitrogen

As there was a lost in compost mass owing to the decomposition of organic matter, the total nitrogen of all treatments are generally increased.(Wong et al, 2009)

From Figure 4.10, control and R4 only had a small increase in total nitrogen which indicates a slow decomposition rate with small loss of dry weight. While for the R2, R3 and R5, the total nitrogen is small or less the same. However, as the degradation rate of R5 is highest, the compost mass would be lower and hence a higher concentration of total nitrogen concentration was given. The total nitrogen of other treatment also corresponded to the degradation rate.

Figure 4.10 Changes of total nitrogen during the food waste composting with different concentration of lime, Magnesium oxide and dipotassium hydrogen phosphate.

4.4.4 Changes of nitrite and nitrate

Nitrate is the terminate form of nitrogen (Wong et al, 2009) that the ammonia was converted to nitrate by the conversion of nitrifying bacteria under an optimum pH 7 to 8. (USEPA, 2002)

Form Figure 4.11 and Figure 4.12, R5 had the highest amount of nitrate and nitrite as it had the highest amount of ammonium retained in the later stage of composting. As nitrate is the final sink of nitrogen, a drop in the nitrite resulted in a rise in nitrate which could shown in R2 and R4.

Figure 4.11 Changes of nitrite during the food waste composting with different concentration of lime, Magnesium oxide and dipotassium hydrogen phosphate.

Figure 4.12 Changes of nitrate during the food waste composting with different concentration of lime, Magnesium oxide and dipotassium hydrogen phosphate.

4.4.5 Change of solid C/N ratio

There was a decreasing trend on the C/N ratio as carbon is lost in form of carbon dioxide gradually during the mineralization of organic matter throughout the composting process. While the loss of compost mass during the decomposition of food waste accounted for the increase of total nitrogen. As a result, there was a decreasing trend for all the treatments as Figure 4.13 shown.

R1 and R4 had smaller reduction in C/N ratio as the low pH inhibit the biodegradation of organic matter and hence not a large reduction of carbon content and large increase in nitrogen content resulted.While the biodegradation of R2, R3 and R5 is fast and hence resulted in a large reduction of C/N ratio.

C/N ratio could also indicate the maturity of compost that the value smaller than 20 was identified as mature. (Hirai et al,1983) Therefore, R2, R3 and R5 were matured as their C/N ratios were under 20.

Figure 4.14 Changes od C/N ratio during the food waste composting with different concentration of lime, Magnesium oxide and dipotassium hydrogen phosphate.

4.5 Crystal formation in compost

Figure 4.15 shows the powder X ray diffraction pattern. There were three sharp peaks in 16,21 and 31 two theta degree. As struvite was a cystal which has a distinctive orthorhombic structure and so it was easily identified by the X-ray diffraction pattern.(Du Xian-yuan et al, 2010) By comparing the powder X ray diffraction pattern of different treatments to the standard as shown in the Figure 4.16, only R5 had the pure struvite formed as the peak of powder XRD pattern in R5 can fit the powder XRD pattern of sruvite standard.

While there might be some impure crystal formed in other treatments that the peak of other crystal may shaded the peak of struvite in XRD patterm and hence it is difficult to determine the type of crystal form.

Figure 4.15 Powder X - ray diffraction pattern of standard struvite

Figure 4.16 Powder X- ray diffraction pattern of different treatments

4.6 Total phosphorus

The initial total phosphorus of different treatments was not the same there were different concentrations of dipotassium hydrogen phosphate added. For R1 and R2, there was no addition of dipotassium hydrogen phosphate, as a result, it had a low initial total phosphorus concentrations. While both R3, R4, R5 also had the dipotassium hydrogen phosphate addition and hence had a higher initial phosphorus concentration. However, R4 and R5 had a higher total phosphorus concentration than that in the R3 as double concentration(0.1M) of dipotassium hydrogen phosphate is added in R4 and R5.

There was an increasing trend for the total phosphorus as the dry weight of compost decreased along with the biodegradation of food waste. As the phosphorus are not volatile, it retained in the compost. Therefore, the higher the decomposition rate the larger the increase of total phosphorus due to the concentration effect. As a result, R5 had a higher TP than R4 even though the same amount of dipotassium hydrogen phosphate was added since the decomposition of R5 is the highest among five treatments.

Figure 4.17 Changes of total phosphorus during the food waste composting with different concentration of lime, Magnesium oxide and dipotassium hydrogen phosphate.

4.7 Phytotoxicity

4.7.1 Electrical conductivity

Electrical conductivity reflects the salinity in compost which was able to indicate the phyto inhibitory effect on the growth of plants. (Ren et al, 2010) 3.00 mScm-1 was identified as the limiting EC for growing plant safely. (Garcia, 1991)

Form Figure 4, all the plants had the EC higher than 3 mScm-1. R4 and R5 had higher EC as larger amount of salt was added in this two treatments. While R1, R2 and R3 had a comparatively lower EC, as they would not pose a serious inhibitory effect to the plant.

Figure 4.18 Changes of electrical conductivity during the food waste composting with different concentration of lime, Magnesium oxide and dipotassium hydrogen phosphate.

4.7.2 Seed germination index

Seed germination is another maturity indicator. According to Tiquia and Tam (1998a), the phytotoxicity is minimal when the germination index exceeded 80%.

From Figure 4.18, both GI in R2, R3and R5 exceeded 80% and hence they were uitable for the plant to growth. However, even the GI in R5 was higher than 80% but it was much lower than that of R2and R3 because of its high electrical conductivity which posed a phyto inhibitory effect on plant growth.

While R1 and R4 showed a very low GI due to a low pH which inhibit the microbial activity and hence giving a slow degradation of organic matter inhibiting the plant growth.

Figure 4.19 Changes of germination index during the food waste composting with different concentration of lime, Magnesium oxide and dipotassium hydrogen phosphate.

Chapter 5 Conclusion and further studies

5.1 Conclusion

The pure struvite only form in R5(Addition of 0.05M MgO and 0.1M K2HPO4 ) has no lime addition, which mean there are interaction between the lime and phosphate which hinders the formation of struvite. Besides, although the struvite form in R5, the amount of nitrogen loss is still higher than that of R3(Addition of 2.25% lime, 0.05M MgO and 0.05M K2HPO4 ). Struvite may not be the best crystal to minimize the nitrogen loss during the food waste composting.

In term of biodegradation, loss of nitrogen and maturity of compost, R3 is the beast among five treatments as it has the fair performance on different aspects.

5.2 Further Studies

Although the optimum pH for struvite formation is 8.5-9.2, but the lime addition cannot enhance the struvite formation and may form other crystal instead of. Further investigation is need to find out the interaction between lime and struvite.

Besides, there are other crystals formed in different treatments such as the crystal formed in R3 which can successfully reduce the loss of ammonia. As a result, a further studies could be conducted to identify the crystal formed in different treatments and investigate the crystal which best minimize the nitrogen loss.