Investigating the role of bulking agents in compost maturity

Kitchen waste is increasing globally, similarly in Pakistan bulk of municipal solid waste comprises of kitchen waste specifically, tea waste. Composting of kitchen waste is one of the promising ways to convert waste into useful product, resulting into zero waste. This study is aimed to convert waste (kitchen waste) in to a resource (compost) using bulking agents (tea waste and biochar) for reducing maturity time. Secondly, compost application on Solanum lycopersicum (tomato) was also tested. Four compost treatments were designed under aerobic composting conditions for 30 days. Tea waste and biochar have accelerated the maturity rate and produced a nutrient rich compost. Final compost had Electrical Conductivity of 2mS/cm, Carbon Nitrogen ration of 15, 54% of organic matter, 15% of moisture content, 48% of cellulose content, and 28% of Lignin content. With the use of Co-compost the Solanum lycopersicum showed 133% germination index, 100% germination, 235% Munoo-Liisa Vitality Index and 1238% seed vigor index. Co-compost also improved the soil total nitrogen by 1.4%, total phosphorous by 2%, total potassium by 2.1% and bulk density by 2.6 gcm−3. This study successfully used tea waste and biochar as bulking agents to reduce maturation time to 30 days. Tea waste and biochar enhanced the organic matter degradation, lignocellulose degradation, water holding capacity, porosity, seed’s vigor, germination index. This research can be helpful in developing home composting and home gardening to combat solid waste management and food security issue in developing countries.


Composting process and sampling
The process was conducted at household level for the duration of 30 days from July 2022 to August 2022.Aerobic indoor composting was chosen for this study 13 .Compost pile was made in pot with a dimensions of 30 × 30 cm (length x diameter).Layers were established to make a compost.KW and BAs were added in 3:1 weight ratio, respectively 20 .10 kg compost pile was created for each treatment.To maintain the moisture of the compost pile 10 ml water was added after every 6 days for initial 12 days.Compost pile was turned every day after noting temperature to maintain proper conditions for composting 21 .Samples were taken on days 2, 10, 20, and 30.Samples were collected each time from three depths (near the surface of pile, midway of pile, and core of the pile) and they were mixed to create a composite sample, samples were then placed in a clean plastic bag at 4 °C to perform further tests 17 .Four treatments were designed for this study with three replicates (Table 1).

Physical and chemical analysis
Electrical conductivity (EC) and pH was determined in a 1:10 (w/v) water soluble extract 4 .Moisture content (MC) was measured by dyring the compost samples at 105 °C till constant weight is achieved 21 .Organic matter (OM) was quantified by loss on ignition (LOI) method in muffle furnace (550 °C) 22 .Total carbon (TC) and total nitrogen (TN) was assessed by automatic elemental analyzer 17 .Cellulose and lignin contents are calculated using reflux method of Keeflee et al. 23 using Eqs.( 1), ( 2), (3), and (4).
Water holding capacity (WHC) and bulk density was calculated using Eqs.( 5) and (6) as reported by Ahn et al. 20 ; where Ms = weight of sample after filtration, M1 = weight of initial sample, MC = moisture content of initial sample.
Porosity was calculated according to Tautmann and Krasny 24 using Eq.(7) given below; where PSV = pore space volume, Vi = volume of initial sample.
Germination index GI was calculated according to Abdel and Al-Shaieny 25 using Eq. ( 8); where Gs = total number of germinated seeds, D = number of days.

Pot experiment
The seeds of tomato were soaked in warm distilled water for 3 h.Pots (30 × 30 cm, length × width) were filled with soil and successive treatment of compost.The pot treatmnets are shown in Table 2. Seeds were then sown into each pot and the experiment lasted for 30 days.Each pot was sprayed with sufficient amount of water daily to maintain the optimal moisture conditions 6 .Growth was measured by counting number of leaves and height of stem.To measure height of stem measuring rod was placed at the bottom and was taken to the top of the stem.
Leaf length was measured from the point where the leaf joins the stalk to the pointy end of the leaf 8 .Leaf width was measured from 1 end to the other at the widest part of the leaf 9 .All the readings were taken in triplicates.

Temperature, pH and electrical conductivity changes during composting
Temperature is greatly influenced by different treatments p < 0.05.Temperature rise in T 3 is very abrupt compared to the other treatments.While the shift in T 4 from thermophilic phase to cooling phase is quite sharp.T 2 showed more gradual temperature change than the other treatments.All the three treatments except T 1 have maintained a very high temperature (50-60 °C) for more than a week (Fig. 1A).T 3 and T 4 showed sharp temperature changes, this might have happened because of the presence of biochar, as it has a great water absorption potential which cause the rapid changes in temperature 27 .High temperature is achieved by the activities of microbes 28 , this rapid microbial growth is facilitated by high surface area of biochar and tea waste 29 .T 1 (having no bulking agent) did not achieved high and long thermophilic phase, which was essential to kill pathogens 30 .Biochar has good amount of OM available which also enhances the microbial and enzymatic activity 31 , resultantly the temperature rises 32 .
During the composting period pH vary between 4 and 8 because of the different reactions happening in the compost pile 33 .Different studies have established that pH drops at the start of composting 24 because of the development of anaerobic conditions 25 and consequently organic acids formations 34 .It is evident in that pH values are also influenced by different treatments p < 0.05.T 4 gradually moves from acidic to alkaline pH while T 3 again shows abrupt changes in pH.On the other hand, pH in T 2 turns acidic in the mesophilic phase and then rise to alkaline gradually (Fig. 1B).In T 1 , pH remained acidic for a very long time even in the thermophilic phase, this might be because of the high organic acid concentration 31 , low mineralization 17 and slow degradation rates of OM/organic N 35 .While in other three treatments pH started to move towards neutral as the thermophilic stage approached.This is because of the aerobic conditions, high temperatures 31 , mineralization of organic nitrogen 17 and degradation of organic matter 36 .EC of the compost represents the salt content of the mature compost 37 .It is an important parameter in terms of explaining the effect of compost on plant growth 38 .EC is directly linked to the pH values as the pH increases EC also increases 35 .EC values of the three treatments T 2 -T 4 are under safe EC range of 0-4mS/cm, while T 1 (no bulking agent) has EC value higher than 4 mScm -1 (Fig. 1C).High EC values present the threat of salinity in the soil which can be harmful or even fatal for the plant growth 25 .High EC range is because biochar release high amounts of soluble ions and have higher potential of accumulating different ions 11 due to great surface area 39 .

Moisture content, water holding capacity (WHC) and porosity changes during composting
Moisture content determines the fate of composting and nutrient content of the compost.Higher MC result in nutrient leaching and it does not allow the temperature to rise.Moisture content of all the treatments was remained in the optimum range, except T 1 (Fig. 2A-C, Table 3).At the start of composting process MC should be in 50-65% range to stop the nutrient loss from leaching 40 .At the end of composting MC must be in 15-25% range and this criterion is not followed by T 1. MC has an indirect impact on the WHC and porosity 41 of final compost (Fig. 2).As MC increases porosity and WHC start to decrease 42 .WHC is significantly different is all treatments p < 0.05.T 4 shows the highest water holding capacity.WHC of compost should be high so that the nutrient loss can be reduced, aggregate size and soil structure can be enhanced 27 .In this study, all treatments WHC are significantly different (p < 0.05) from each other.WHC of T 1 is very low while T 4 has highest WHC, this is due to the higher surface area and adsorption capacity of bulking agents 3 .Porosity is also an important parameter which influence soil quality and plant growth.This is also dependent upon the moisture 21 content and bulk densities 20 .Optimum conditions of porosity and WHC are directly proportional to the type and quantity of bulking agent 42 .

Organic matter content and C/N ratio variations in compost
OM is one very essential parameter to determine the maturity of compost.Different treatments are significantly different (p < 0.05) for OM (Fig. 3A).T 4 showed the highest degradation rate and high OM at the start of composting process.T 2 and T 3 showed comparatively slow degradation rates (Fig. 3A) and it declined with the passage of time.T 1 is considered immature compost because the OM content was only 16% and to be a mature compost, the OM content should be around 30-50%.OM content keep on decreasing as it is used by microbes as energy 43 and food source 35 .Organic C is also lost during composting through volatilization in form of CO 2 44 .Low degradation rates are low in T 1 because there is no bulking agent to support the growth of microbes 45 .TW and BC both promoted OM degradation due to their large surface area for the nurturing of microbes 6 .High adsorption capacity of bulking agents provide sufficient nutrients for microbial community 46 .Biochar promotes the enzymatic activities and is able to decompose even the insoluble OM in the compost pile 47 .Optimum C/N is also very important parameter for successful composting and is a potential indicator of compost maturity 48 .Different compost treatments p < 0.05 are significantly different from each other in C/N ratio.The C/N ratio of T 1 is higher than 18 and this can cause excessive mineralization or immobilization of N, which is not beneficial for the plant growth.All the other three treatments (T 2 , T 3 and T 4 ) are under 14-18 range which is indicative that compost is matured and can be used to enhance plant growth (Fig. 3B).
In order to determine the carbon and nitrogen content of the compost the ratio of waste to bulking agent should be adjusted 40 .Appropriate C/N ratio is vital to sustain the microbial population in the most active form to accelerate composting process 49 .High C/N ratio can cause delay in maturation period while lower C/N can cause rapid N losses through volatilization or runoff.C/N ratio of final compost should be 14-18 which ensures slow mineralization of N when applied to the soil 48 .

Cellulose and lignin degradation
Lignin is mostly degraded by mesophilic bacteria in presence of less OM, thus it is degraded mostly in cooling phase (Fig. 4A,B).Statistical analysis revealed that all the treatments are significantly different (p < 0.05) from each other (Fig. 4A,B).T 4 showed excessive degradation (28-9%) potential because BC and TW offered high surface functional groups and optimal conditions for enzymatic activities 50 .Similarly, the absorption and strength of enzymes increased due to microbial abundance in BC and TW 51 .Cellulose content was high in the start of the experiment and at gradually decrease with the passage of time (Fig. 5A,B).The maximum degradation (73%) was observed in T 4 and all the treatments differ significantly different (p < 0.05) Cellulose degradation is vital for both compost maturity and bioavailable of nutrients for plant growth 52 .

Micronutrients in compost
NPK is an essential parameter in compost maturation as this increase the plant growth.In present study, all four treatments have different nutritional value (p < 0.05) (Table 2).T 4 have shown enough NPK while T 1 did not meet the target.T 2 and T 3 have almost the same amounts of NPK.These results are in agreement with the pot experiment where stem height and number of leaves were increased in all three treatments compared to the control, while T 1 showed slightly lower stem height than control treatment.Higher ratios of NPK can also cause immobility of the nutrients into the roots of plants 11 .Lower NPK ratios also can cause nutrient deficiency in the plants 47 .

Germination index and germination percentage
Phytotoxicity determination is a crucial parameter to check the compost maturity 53 .The present study, has performed different experiments to check whether the compost is matured enough or still contain phytotoxic substances.Compost is considered mature when it has GI higher than 80% 3 .GI experiment was performed in four treatments and they were significantly (p < 0.05) different.G1 was 66% which indicates the presence of phytotoxic substances in the compost and it is not yet matured (Fig. 6A, Table 2).GI of other three treatments is in the range of 116-133%, this is due to the combined adsorption properties of TW and BC.Bulking agents also  provide adsorption of volatile 33 and toxic substances 50 .G% indicates the germination rate of the compost and G 4 has achieved germination rate of 100% that shows all the seeds have grown (Fig. 6B).This is because that TW has the ability to produce humic substances 14 , which make root cell membrane structurally more stable 17 .Humic substances also make cell more permeable to take up nutrients 15 and resultantly enhance the plant growth 54 .The mechanism for BC is different, it has high water retention potential that results in slow mineralization 55 and less leachate production 53 .This results in reduction in nutrient loss 43 and consequently increase the plant growth 46 .

Seed vigor index and Munoo-Liisa vitality index
Seed vigor index (SVI) shows the ability of the seeds to grow in the provided conditions, it is directly proportional to the crop yield 55 .The present study reported p < 0.05, which means that all treatments have different capability to facilitate.G 4 showed highest SVI of 1238 and the lowest vigor values are found in G 1 (Fig. 7A).High SVI is also linked with high phosphorus content 29 and nutrient availability 32 .Likewise, high porosity and high WHC of compost facilitate the seed growth 27 and results in high SVI 40 .Munoo-Liisa Vitality Index (MVI) in an important indicator that shows how much crop yield can be produced using a specific amendment 56 .The standard value of MVI is set at 80% by different agricultural departments, internationally 26 .In this study, G 1 did not match up to the set target while G 4 showed the highest yield percentage (Fig. 7B, Table 3).Values lower than 80% are indicative of the fact there are potential phytotoxic substances present in the compost 54 .

Practical implications and future prospect
This study has successfully established that biochar and tea waste can be used in kitchen waste composting as it is not harmful along with its milk content.This study will help promote organic farming as the compost is cheaper and have the high nutritional value and can replace chemical fertilizers that are expensive and are harmful for long term use (Table 4).Secondly, most of the farmers and commercial composting plants does not use kitchen waste that is mixed with tea waste because of milk that has the non-vegetarian origin.Kitchen waste co-compost can be an economical method for reducing food security (Table 4).Kitchen waste compost maturity is a serious challenge and as per the findings of this study it can be best dealt with using biochar.It has a good capacity for water adsorption and consequently generating pressure which is good for heating up the composting system.It has been established that composting with bulking agents and raw material is a more sustainable option than incineration and open dumping.Rather throwing kitchen waste, we can easily convert this into a useful resource (compost).In developing countries, low income groups can use this compost to grow vegetables in home/kitchen gardening.This can be helpful to overcome supply shortage and food inflation.

Recommendations
• Tea waste should be incorporated in kitchen waste composting in order to minimize the amount of solid waste produced.• Application of compost to soil and its impact on plant growth and soil quality for a longer period of times should be studied.• Composting of kitchen waste is an inexpensive method compared to the chemical fertilizer application.
• More studies should be carried out in open fields with different variables and optimization techniques.
• The findings of this study will help promote kitchen waste composting with tea residue without producing harmful impact.• The time barrier for kitchen waste composting has been reduced using tea waste (another waste).So, this study provides a feasible option to recycle both kitchen and tea waste effectively.

Conclusion
The present study showed that tea waste is not harmful along with milk contents, it can be used in composting of kitchen waste.The study showed that teas waste and are effective bulking agents and created positive effect on maturation of kitchen waste compost.They improved the compost quality by enhancing organic matter, lignocellulose degradation, water holding capacity, porosity and plant growth.Compost get matured in 30 days and can be used as organic fertilizer.The combined addition of both bulking agents enhanced the growth of tomato plant.This study successfully met its objectives and recommended a practical method for accelerating kitchen waste composting.Further studies must be carried out to study the long-term impact of organic compost on plants.Long maturation time is a serious challenge in kitchen waste composting and this study effectively reduced the maturation time by using biochar.Composting with bulking agents and raw material is more sustainable option than incineration and open dumping.The results of this research can be used for home composting and successively in home/kitchen gardening.This can be beneficial in reducing the municipal solid waste, reducing burden on landfill sites, converting waste into a resource.In developing countries this home composting and home gardening can also help to overcome food security issues.
Bulk density(g/cm 3 = weight of the sample(g) volume of sample(cm 3 ) Germination index(GI) = Gs D

Figure 1 .
Figure 1.Changes in parameters during composting process, (A) Temperature changes, (B) pH changes and (C) Electrical conductivity.

Figure 2 .
Figure 2. Correlation between different variables, (A) Water holding capacity and porosity, (B) Water holding capacity and moisture content, (C) moisture loss in different treatments.

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Compost treatments and their composition.
26l the data is presented in mean values of three samples.The data was analyzed using Microsoft Excel 2016 (Microsoft Corporation, USA) and SPSS Statistics 16.0 software.Post Hoc test, Tuckey test, descriptive analysis, least significant difference LSD, One-way and Two-way ANOVAs were performed26.

Table 3 .
Maturity indices of final compost.

Table 4 .
Comparison table of studies on different compost types.