Households as Sources Contributing the Most to Urban Solid Wastes Production: Implications for a Better Management and Disposal

Households are the sources contributing the most to the production of urban solid wastes in developing countries. The present study analyses the generation of domestic solid wastes in a settlement in Mexico in two different years. The goals were to compare composition and rates of generation into the periods to assess changes and the influence of socioeconomic variables. In 1998 and 2004, wastes from households were sampled of three socioeconomic levels randomly selected, and a survey was applied. Data obtained were analyzed with descrip-tive and multivariate analysis. Results of generation rate and composition of domestic solid wastes showed a negative correlation between generation and income in the first one, but positive in the second one; in both, composition did not show correlation. Rates of per capita generation, per household, population and density of inhabitants per dwelling increased, and the generation of wastes were more with respect to population growth, which emphasizes the influence of economic and demographic variables. This study corroborates that households are the principal sources of urban solid wastes and it is emphasized that management strategies must preponderate households to be successful. In addition, this information is important to establish strategies for an integral management of urban solid wastes in developing countries.


Introduction
The generation of urban solid wastes (USW) represents a severe problem for the environmental authorities of Mexico, as for the majority of developing countries [1]. USW are composed of a mixture of heterogeneous materials that are by-products inescapable of production and consumption patterns of contemporary societies [2]. The design and implementation of efficient programs for improving current USW management systems are urgent challenges that must be addressed by urban authorities in short-time. Deficient technical and operational structures at local and national scales are reflected frequently in an insufficient USW collection together with an inadequate disposal [3]. Furthermore, limited or null data for generation and composition of USW also contribute to limit for predictions that could help to better design collection policies [4] [5].
The lack of updated and precise data of USW generation is related to the rapid change in generation rates and composition. Such changes are strongly linked to variations in population lifestyle and consumption patterns. In economically developing countries, increases in household income, urbanization, and spread of more manufactured products clearly influence both generation rates and composition of USW [6]. These rapid changes also contribute to low efficiencies of USW collection and management systems, that when coupled with bad design and outdated policies such as the lack separation and recycling operations result lately in waste of economic resources [7] [8]. All these led to social, sanitary and environmental issues for urban authorities that should provide services for a growing population. Moreover, most cities should deal with a growing vulnerable population that operates irregularly collection and separation services as a way to obtain economical resources.
For several decades households have been increasing their contribution to the generation of USW in urban areas of developing countries [9] [10]. For instance [11] reported a contribution of households of around 87% to USW for metropolitan areas of Mexico. Existing studies for domestic solid wastes (DSW) and USW made during the 1990s reported that composition and generation depend significantly on demography [12], socio-economic conditions [13], perception and behavior of population [1] and, individual and family consumption patterns [14]. The above highlight the need for precise and updated composition and generation rates of DSW, which could be used to design efficient management programs that must include valorization and recycling operations but also an adequate confinement of heterogeneous solid wastes [15].
This study addresses changes in composition and generation rates of DSW in the city of Morelia, Mexico, during two different periods, which allows identifying variations of by-products and the influence of socio-economic variables in composition and generation of these wastes. Because households are keys in the USW generation route as observed in most cities of developing countries, updated information of composition and production of wastes from households, is The above, confronts the urban authorities with serious problems to provide efficient urban services, among the most pressing are the water supply and the collection service for a growing generation of USW, which is reflected in an inadequate provision of these, among other environmental problems. Urban authorities in Mexico experience typically must operate with reduced funding to provide an efficient collection of USW and water and electricity supplies. The results presented here provide both statistical information and evolution of composition of DSW through time.

Sampling and Characterization of DSW
In the studies of 1998 and 2004, the sampling of DSW was made during seven days, prior notification to all selected household inhabitants to ratify their collaboration in the study and the number of inhabitants in each home. Samples of DSW were collected in plastic bags where waste produced during periods of 24-h were placed. After collection of DSW, the bags were weighted and labelled on a daily basis and socio-economic strata, data were captured on a daily basis according to Table 2; the characterization of samples of DSW was made following the method cited in [17]. Table 3 presents some key data from the studies car- Samples were prepared through the quartering official standard norm (NMX-AA-015-1985) [18], and DSW was sorted according to the official standard norm NMX-AA-022-1985 [19]. Briefly, such standard establishes that DSW samples must be weighted, and then separated into by-products which are sub-     [21]. A demographic projection of 2012 is shown, since we made another study of DSW generation different from the methodology previously used, however, the samples were taken from the truck that provides the service in the selected suburb.    [11].  Other a : ferrous material, polystyrene, ceramics, feminine towels, flasks, ash, diaper, stone, colour glass, polypropylene, aluminium, hazardous waste, construction materials, polyvinyl chloride, leather, tetra brick, cotton, electronic waste, coal, rubber, wax paper, batteries, laminate bag, wax, fine residues, metallic paper, flour, polyurethane, wax cardboard, stool, cellophane paper, butts, broom, clay, detergent, viscera, hair, animal hair, feathers, filters, latex, straw, eraser and laminated paper.

Discussion
The USW increasing generation in economically developing countries such as Mexico suggests the influence of economic growth, demographic changes, modifications in consumption patterns and lifestyles of the Mexican society. These are related to the overall change from a predominantly rural lifestyle during the Others a : ferrous material, polystyrene, ceramics, feminine towels, flasks, ash, diaper, stone, colour glass, polypropylene, aluminium, hazardous waste, construction materials, polyvinyl chloride, leather, tetra brick, cotton, electronic waste, coal, rubber, wax paper, batteries, laminate bag, wax, fine residues, metallic paper, flour, polyurethane, wax cardboard, stool, cellophane paper, butts, broom, clay, detergent, viscera, hair, animal hair, feathers, filters, latex, straw, eraser and laminated paper. ND: No determined. Journal of Environmental Protection first half of the twentieth century to a predominant urban one since the 1970's.
This coincides also with the Mexican Bureau of Census (INEGI) data that reveals around 70% of the population of Mexico is located in urban settlements [21].
Results of the study conducted in 2004, it is interesting to note that the low stratum produced on average 25% less food wastes than the other two strata, while the presence of diapers was detected in all socioeconomic strata. However, the number of diapers founded in the low stratum was four times higher than in the other two strata, which confirms that income and population growth are the predominant factors in the changes in the composition of DSW. About the above, it is noteworthy that the number of diapers shows an inverse relation with the rest of residues from the consumption of goods, since the study carried out in 2004 showed a decrease of 10% of these with respect to 1998. The above also confirms the reduction of the birth rate as part of the demographic changes, which is more evident in the middle and upper strata, since the low stratum continues with birth rates similar to the 1980s, and increases in teenage pregnancies, with which Mexico has the first place among the Organization for Economic Cooperation and Development countries (OECD) [22].
The modification of consumption patterns and lifestyles of the urban population under study is basically confirmed by changes in the composition of the following byproducts: a) presence of diapers in the DSW stream, since the use of disposable diapers is already considered as a basic consumption item for babies and also the occurrence of adult diapers shows an increasing tendency in the two sampling periods; b) food wastes, the generation rate of this item in the low socioeconomic stratum between the two years of study verify the reduction of food wastes; c) yard and vegetable fiber residues increased, which makes it necessary to implement the collection of sorted wastes and reduce clandestine disposal.
The percentages of fiber residues confirm the permanence of corrals and patios within the peri-urban zone, especially in zones of the low stratum; because the growth of the city has absorbed rural areas that continue with agricultural activities.
The quantity of sanitary paper founded (4%) is of critical interest in USW management due to the health implications of the exposure of fecal material to the workers and informal collectors. In Morelia, there are still factories of corrugated cardboard sheet that demand sanitary paper as raw material, which is separated by these people with the consequences of exposure to fecal matter. On the other hand, the disposal of sanitary paper within the drainage system is not feasible either, since although the city has a wastewater treatment plant, the drainage system is insufficient and obsolete.
The increase in the amount of DSW generated by dwellings in Morelia is a serious challenge for the public cleaning systems, since implies an increase in budget for the collection and landfilled of the wastes. In Mexico in 2017, 102,895 Tons of USW were generated daily, of which 83.93% were collected and 78.54% landfilled, recycled only 9.63% and dumped around 11% [23]. In Morelia, the landfill is eight years old, but in order to increase its useful life to another 10 years the extension of the area is contemplated, since generation rate of DSW has increased at 22% between the two study periods of 1998 and 2004 and 28% according to the 2012 study. This last fact, although not obtained from the methodology of the two previous studies, was done by taking the sample of DSW from the collection unit of a randomly selected suburb. Data confirm a growing tendency of waste generation in households, so this highlights the need to promote studies to locate facilities for the final disposal of solid wastes, which comply with environmental legislation, as well as increasing recycling and biological treatment to reduce the amount of wastes landfilled. In addition, having updated data on the composition of the DSW allows implementing strategies to improve the separation of wastes in dwellings. The results obtained in this study provide important data for municipal authorities and decision makers to implement management programs that include the separation of wastes in households and legislate to establish in these sources the polluter pay principle, and like so, recycling of sorted materials.
The management of the USW in cities such as Morelia, located in countries with economies in transition and/or developing countries, faces changes owed basically to demographic changes, consumption patterns and lifestyles. This has already been reported in various investigations such as [24] [25] and [26]. According to [27], the integral management of USW, also focuses on the improvement of the processes to optimize the physical, chemical and biological stability of the confined wastes, and on the valorization of the components to increase the rate of recovery and recycling in order to lengthen the time of useful life of landfills and as consequence diminish the costs of operation and post-closure, as well as of the impacts associated with the management of these facilities. The implementation of these measures involves the payment of economic resources, which is the main constraint that municipalities face. However, it is necessary that these strengthen the integral management of the USW, which must predominate actions and programs focused to prevent and minimize the generation of DSW; improvement of collection and transport of the USW by acquiring enough and adequate collection trucks which fit with the urban design of the cities and of the characteristics (composition) of the USW. Added to the above, the construction of transfer stations in cities of more than 100,000 inhabitants, it is a very important issue. These must be designed with a sufficient capacity, security measures and above all, that the facilities must not be built in the same place of the landfill, since this increases transportation costs of the USW. Finally, it is very important summarizing that all the measures that municipalities undertake, should include information to the population on measures to reduce the production of solid wastes in dwellings, since this study corroborates that these sources are the ones that contribute mostly to the generation of USW.

Conclusions
An increase in the production of DSW between the periods of study is observed, The methodological difference between sampling in the third study does not allow concluding about the generation rate and composition of DSW is determined by income. From the above, it is important to continue with the monitoring of these parameters as well as to expand the number of samples.
Notably, there was an increase in the generation of wastes derived of plastics and a reduction from the glass, which suggests that in addition to consumer demand, the prices of raw materials also affect the decision of manufacturers to develop packages and containers and, consequently, in the composition of DSW.
The increase in the generation of DSW faces to municipalities to provide a greater coverage by the collection systems and the construction of facilities to landfill the DSW.
The amount of food and sanitary wastes continues to be high; however, it is necessary to implement other disposal measures, different from drainage, since its implementation in developing and transition countries is not economically feasible because high costs that implies the construction of water treatment plants and modernization of drainage system. The differentiated collection of these wastes can be the best solution.

Absolute Recommendations
It is absolutely necessary to implement programs to encourage the separation of wastes in households and the recycling of sorted materials. The above reduces the cost of waste collection and disposal, as well as the environmental benefit due to the reduction of demand for raw materials.
Decision makers and politicians must achieve recovery of sorted materials to reach the zero generation. Circular economy is the axe which must transit the systems of municipal solid waste collection in developing countries.