DEVELOPING COMPOSTING PROCESS SCHEME FOR PORTHARCOURT, NIGERIA Awajiogak Anthony Ujile* and Emmanuel Ehirim E-mail:
[email protected] Tel: +2348033398876 (for correspondence) * Department of Chemical/Petrochemical Engineering, Rivers State University of Science & Technology, P. M. B. 5080, Port Harcourt, Nigeria ABSTRACT A composting process scheme for Port Harcourt Nigeria is developed. The composting facility required for the process is not only designed such that technical viability is achieved, but also to handle sufficient volumes of wastes generated in the metropolis. Principles of bio-kinetics, mass transfer, and heat transfer and equilibrium phenomenon are applied for the behavior of pathogens in the process schemes. Sizing of the composting facility depends on these basic engineering principles. The processing capacity of 225 tons of municipal solid waste per eight- hour shift, for two shifts a day for seven composting plants distributed at strategically located zones are suitable to solve the problems associated with waste generation in the city. Determination of the compost recipe, monitoring and parameter (temperature, odor management, moisture, oxygen and carbon dioxide, monitoring equipment) adjustment are presented as part of the process scheme. The choice of the composting method is a function of operational cost and the results obtained from the analysis of the parameters .
Keywords: waste utilization, active aeration, bacterial activity, aerobic decomposition and ultimate BOD INTRODUCTION Composting is the controlled aerobic biological decomposition of organic matter into a stable, humus-like product called compost. It is essentially the same process as natural decomposition except that it is enhanced and accelerated by mixing organic waste with other ingredients to optimize microbial growth. Efficient composting requires that the initial compost mix have the following conditions [1]:
A balance source of energy (carbon) and nutrients (primary nitrogen), typically with a carbon- tonitrogen (C:N) ratio of 20:1 to 40:1 Sufficient moisture, typically 40% to 60% Sufficient oxygen for an aerobic environment, typically 5% or greater A pH in the range of 6 to 8
These compost mix characteristics must be maintained throughout the composting process in order to achieve high efficiency. Presently, Port Harcourt has no controlled biological decomposition of organic matter into compost. This means that the government is losing the potential/economic benefits of composting manure. The present situation where the municipal solid waste (MSW) is disposed without control is not only causing menace to the environment, but also constituting to economic loss to the GPD. As at the year 2007, 7500 tons/day of MSW were generated and out of these quantities 41% (organics) were compostable [2]. Composting of vegetable and fruit wastes tends to fail under aerobic metabolism if improperly managed [3.4.5]. Presently, there is no plan or scientific procedure in place to utilize this portion of the generated wastes. Technique of feeding substrate daily into batch culture at a constant rate known as feed batch operation had been investigated in Thailand [6].
The aim of this work therefore is to provide a systematic composting plan for the city of Port Harcourt. Port Harcourt is located within the Niger Delta region at the southern most part of Nigeria. It is bounded by Longitude 6o 56’ to 7o 07’ East and Latitude 4o 44’ to 4o 52’ North of the Equator see figure 1
Fig. 1 The Port Harcourt Metropolis [7] The metropolis requires a detailed procedure as to incorporate all measuerable deliverables. Therefore the principal parameters considered in planning a compost facility for the area include: soil investigations, developing recipe design, facility design, waste utilization plan and operation and maintenance plan. SELECTING A COMPOSTING METHOD There are four general composting methods which may be used to compost organic tissues: Open pile, Bin, Windrow, and In-vessel. Each requires orderly formation, identification and management of composting batches.Branton, [8] asserted that passive, forced and (or) active aeration may be used with each other for optimum performance. These methods vary in their speed of composting, environmental risk, expense, and aesthetics, therefore considerations made on choice are based on high efficiency. The Bin type is chosen for the Port Harcourt metropolis for the following reasons:
easy to manage batches and greater control of composting process; if roofed, little or no risk of precipitate leachate; existing facilities or materials may be used; easy to designate batches; makes use of existing farm machinery; greater control of microorganisms. COLLECTION OF SOLID WASTES The present collection services being used should be modified to a large extent. Ujile [9], presented an integrated approach which involved sorting of the wastes before disposal. The sorting process should actually start from the generation point. As wastes are generated they should undergo sorting as to identify the compostibles. If the oganic materials, excluding plastics, rubber and leather, are separated from MSW and subjected to bacterial decomposition, the end products remaining after dissimilatory and assmilatory bacterial activity are the compost [10]. METHODOLOGY The principle of Biochemical Oxygen Demand (BOD) exertion on the compost is applied. A relation between the amount of BOD exerted on compost at time ,t and ultimate BOD is given by the relation [11]: Yt = L (1- e-kt )
(1)
Where k = reaction rate constant (base e, days -1 ) L = ultimate BOD (mg/L) t = time (days) Yt = the amount of BOD exerted at time, t (mg/L) Engineering design requirements for thermal destruction facilities are needed for the following conditions: (1) The rated capacity of the facility, in tons/day and tons/hour and the maximum gross heat release rating for each boiler/incinerator. (2) Establishment of a method to allow the measurement of the rate of waste charging to the individual combustion unit(s), averaged for each over a discrete 24-hour period. In the case where the thermal destruction facility recovers energy for use by means of steam production, the boiler system and its auxilliaries may be used as a calorimeter and the following parameters factored into the method of determination:
Direct measurement of salient variables where such are available Adjustment of parameters to account for variability in unit thermal efficiency as equipment is cycled for maintenance. Seasonal variability of the higher heating value (HHV) of the waste subject to combustion.
(3) Project average and peak daily deliveries of waste to the facility and charging rates to the combustion units (given in tons and estimated volumes). Quantify seasonal trends when anticipated. (4) Designate normal loading, unloading and storage areas to be employed in the facility’s handling of incoming wastes to be processed and residual materials generated by facility operations. This principle can be applied to recover energy as well as compost from solid wastes. However, in this work we utilized the cold process of obtaining compost from waste. Considering the Bin type composting unit, we applied the Newton’s law of cooling. Convection is determined using convection coefficient (heat transfeer coefficient), h with the following relation: Q = h A (Tw - T∞)
(2)
Where A = heat transfer area Tw = work temperature T∞ = bulk fluid temperature The resistance due to convection is given by, R = 1/h A
(3)
The mass transfer coefficient considered is of the form, (N/A) w = Km ∆C m
(4)
Where the use of friction factor (f) to predict heat transfer and mass transfer coefficients (turbulent flow) is shown as: 2
jH
Nu
Pr3
( RePr)
(5) 2
jm
Sh Sc3 ( Re Sc)
(6)
Critical observations of equations 2, 3 and 4 show a common parameter A, surface area for convectional heat transfer, resistance due to convection and mass transfer respectively. Facility sizing depends on these equations, while the biokinetics of the process depend on equation 1. OPERATIONAL COST ANALYSIS A production cost is associated with the land occupied by the compost operation. The value and amount of land available influence the type of composting method used. (Robert,et al 2000 ) asserted that 1 acre of land can handle anywhere between 2000 to 10000 cubic yards (1528 m3 to 7640 m3 ) of compost per year. The In-vessel system or bin considered in this work is based on the scarcity of land in Port Harcourt metropolis. The concept is that the composting material is contained in a building, reactor, or vessel. Proper sizing makes the management and operation of the composting process easier. The sizing procedure involves five steps which we modified to suit our purpose as shown in table 1.
TABLE 1: SIZING PROCEDURE STEPS FOR COMPOSTING PROCESS STEP A B
C
D E
DESCRIPTION Determine the average daily weight of organic waste to be composted (200-250 tons) Determine the composting cycle times for the design weight 1) Primary cycle time (days) = 5 x (design weight)0.5 , minimum time ≥ 10 days 2) Secondary cycle time (days) = 1/3 Primary cycle time, minimum time ≥ 10 days 3) Storage time (days) = Year’s maximum period of time between land application events. Determine the needed composter volumes: 1) Primary composter volume (m3 ) = 0.2 x Average daily loss (kg/day) x Primary cycle time (in days) x 1/(density of compost) 2) Secondary composter volume (m3 ) = 0.2 x Average daily loss (kg/day) x Secondary cycle time (in days) x 1/(density of compost) 3) Storage volume (m3 ) Determine the dimensions of the compost facility including bin dimensions and the number of bins and area requirements Determine the annual saw dust requirement for the composting system.
The cost implication depends not only on the sizing of the composting facility, but also on the optimum performance of the system. It is therefore important that the compost mix is monitored and appropriate adjustments made throughout the composting period to sustain a high rate of aerobic microbial activity for complete decomposition with minimum odour as well as maximum destruction of pathogens. In order to achieve this a convenient and meaningful compost parameter to monitor is temperature. Equation 2 defines heat transfer coefficient with working temperature and bulk fluid temperature. These parameters enhance the determination of optimum composting temperature. Besides, the dimensionless numbers in equations 5 and 6 can simplify temperature monitoring. The details of the application of the dimensionless numbers to monitor the compost temperature may be the subject of another research work on optimisation of composting process. PARAMETER MONITORING AND ADJUSTMENT Other parameters which deserve monitoring and control include: odour, moisture,oxygen and carbon dioxide. Odor: This is the most effective and simple indicator of whether the pile conditions are aerobic and also to a certain degree, if nutrient losses are occuring through ammonia volatilization. Odor management is an important aspect of the composting operation, particularly if the operation is in close proximity to neighbours odors diappear, after the material is incorporated into the compost pile (the odors are masked by the other materials in the pile or eliminators because the microbes in the compost mixture use the odorous compounds as substrates). Strong putrified odors that sometimes smell like sulphur indicate anaerobic activity, particularly when these are accompanied by low temperatures. Most times anaerobic conditions generally develop in response to high moisture and low porosity environments. If excess moisture is not the cause, then the pile may be too large, leading to compaction and inadequate aeration, or the porosity of the material is insufficient. Odor detection is subjective and therefore difficult to quantify or measure. However, if odors have developed, the best solution is to modify the conditions within the pile so that odour production is not continued.
Moisture: The maintenance of proper moisture can be a problem for a composting operation. The moisture conditions in the pile vary constantly throughout the composting period mainly because of the large amounts of evaporation and the addition of water through precipitation. Improper moisture not only slows or stops the composting process, but also leads to aerobic conditions and enhances production. A dry pile is not only detrimental to microbial activity, but also forms dust that carries odors and possible fungal pathogens such as Aspergillus fumigattus. Therefore maintaining the moisture level between 40 and 60 percent alleviates these potential problems as to circumvent the odor production. Oxygen and Carbon Dioxide : Oxygen levels within the pile can also be used as an indicator of how the composting process is devoloping. As aerobic activity increases, the oxygen consumption should also increase causing the oxygen levels to decrease. Measuring oxygen levels to monitor the composting process may not be accurate as measuring temperatures. Oxygen monitoring is most useful to show that stability has been reached. Oxygen levels remain low during the active composting period. However, as the pile reaches maturity and microbial activity begins to slow, oxygen levels rise. Because carbon dioxide is a product of aerobic respiration, it can also be used as an indicator of microbial activity. The carbon dioxide levels should increase as microbial activity develops and decrease as the composting process approaches maturity. Determination of Compost Stability: Compost stability has implications for its curing and use. A stable and mature compost is one that has completed the active composting period and has cured sufficiently. The use of immature compost for potting media or for land application can damage or kill the plants because of excessive C : N ratio, ammonium- nitrogen, volatile organic acids, or other phyto-toxic compounds. Therefore a reliable test of compost maturity is required to prevent any damage that may be brought about by the application of immature compost. A simple and inexpensive test for determining compost maturity is the Dewar self-heating test (Brinton et al, 1994). Fig 2 shows building of compost.
Fig. 2. Typical composting building.
CONCLUSION
NOMENCLATURE Q – quantity of heat required Yt - the amount of (BOD) Biochemical Oxygen Demand exerted at time t, (mg/l) L – ultimate BOD, (mg/l) t – time, (days) k – reaction rate constant (base e, day-1 ) R- resistance due to convection h- heat transfer coefficient, N- mass flux, Km – mass transfer coefficient, ∆Cm – change in concentration of compost Pr – Prandlt Number Re – Reynold’s Number Nu- Nusselt Number Sh- Sherwood Number
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Robert E. G., Gwendolyn M. H., Donald, S. (2000): Composting. Part 637. Environmental Engineering. National Engineering Handbook, United States Department of Agriculture Ujile, A. A (2007): Modeling Leachate Transport in a Sanitary Landfill System- Niger Delta as Case Study. Journal of Solid Waste Technology & Management. Vol. 33. No. 4. Nov 2007. Dept of Civil Eng, Widener University, Chester, PA 19013-5792 USA. Ujile, A. A. (2008): Technique of Municipal Waste Management in Port Harcourt, Rivers State, Nigeria. Conference Proceedings. 23rd International Conference on Solid Waste Technology & Management. March 30- April 2008, Philadelphia, PA, USA p 473-482
