on access to safe and clean water, it is inevitable to bring up public-private partnerships and tariff into spotlight, as they are one of the main factors determining one's ability to access water. a before after comparative study by world bank analyzes how access, quality of service, operational efficiency and tariffs have evolved under 65 public-private partnerships for urban water utilities in developing countries. the study estimates that "ppp projects have provided access to piped water for more than 24 million people in developing countries since 1990". therefore, ppp had been a key significant in helping improved service quality of access to water. apart from ppp, tariffs are also another important factor that directly impact one's access to safe and clean water. in some countries, water privatization can hinder the accessibility of water. when for-profit companies invest in the water system, the desire to make returns on the investment can create a top-heavy distribution system. they increase tariffs to earn more profits, which consequently reduces the accessibility of the resource for poor households since the poor are not able to pay high tariffs. in other words, investments are only made to improve accessibility in richer districts where the people can pay the tariffs. in this manner, the water company's need to make adequate returns is met by supplying water only to those who can pay. however, in other countries such as nigeria and ghana where the governments fail to distribute access to water to the people, water privatization led to expansion of services to low-income districts. impact on health one of the most effective measurement to analyze the effectiveness of water privatization is child mortality rate since children are more likely to be negatively affected by contaminated waters. water privatization has historically had mixed impacts on child mortality and the overall health of the people affected by it. a study of water privatization's impact on health finds that, between 1991 and 1997 in argentina, areas where child mortality was upwards of 26% fell to just under 8% after water was privatized. this occurred due to the regulations private water companies were held to which were more rigorous than their government-controlled counterparts. along with this, development of water infrastructure in impoverished areas at the hands of private companies also positively affected child mortality rates. governments are prone to privatize water companies, among other reasons, to better the quality of the water provided to the country's citizens. in argentina, water privatization did not fulfill many promises the citizens were expecting. this includes the expansion of sewerage treatment and connections and the reduction of the price of water, which actually increased. along with this, the private water companies in argentina needed help from the argentine government to bypass regulatory agencies after it treated to cancel their contract due to conflicts of interest. it is also worth noting that many worker unions were opposed to privatizing water but their pleads were largely ignored by the argentine government. the impact of water privatization on the amount of carcinogenic found in water is highly debated. in some cases, such as the case in the state of north rhine-westphalia, germany, public water systems are likely to invest more money into making water quality good. water companies working on a commercial basis might find it too costly to implement systems to better the water quality beyond what is necessary by law. thus, posing a greater threat of containing harmful cancer-causing substances in the water. impact on tariffs although the impact on tariffs cannot be fully concluded since each country has different policy on tariffs, water tariffs tend to be increased under privatization. for instance, in buenos aires and in manila, tariffs first declined, but then increased above their initial level; in cochabamba or in guyana, tariffs were increased at the time of privatization. however, there are some other cases that tariffs under water privatization did not increase in a long run, typically in sub-saharan africa, where most of investments are funded through development aid. for example, tariffs remained stable in senegal, while in gabon they declined by 50% in five years (2001–2006) and by 30% in ten years in côte d'ivoire (1990 to 2000). in addition, initial tariffs have been well below cost recovery levels in almost all cases, sometimes covering only a fraction of the cost of service provision. the magnitude of tariff increases is influenced by the profit margin of private operators, but also to a large extent by the efficiency of utilities in terms of water losses and labor productivity. however, comparing water expenditure between private and public management in the u.s., a study of household water expenditures in cities under private and public management in the u.s. concludes that "whether water systems are owned by private firms or governments may, on average, simply not matter much." impact on efficiency according to a world bank study in 2005, the most consistent improvement made by public-private partnerships in water supply was in operational efficiency. the study reviews the impact of private management on the efficiency of water utilities in many countries from many continents including africa, latin america, asia, and eastern europe. most evidences from the study suggests that "there is no statistically significant difference between the efficiency performance of public and private operators in this sector.". in addition, a 2008 literature review by the asian development bank shows that of 20 studies reviewed, only three show concrete evidence on technical efficiency improvements or cost reductions under private management. therefore, by 2005, private operator, at least, made an indirect contribution to financing by improving efficiency, making it possible for utilities to finance investments internally instead of having to rely on more debt. profitability an empirical study of 34 concessions in nine latin american countries during the 1990s, including 10 water concessions in 5 countries (3 in argentina, 1 in bolivia, 1 in brazil, 3 in chile and 2 in colombia), has estimated the profitability of concessions compared to the cost of capital of private companies. according to the study, contrary to public perception, the financial returns of private infrastructure concessions have been modest. the average annual return on capital employed was 7 percent. for a number of concessions, the returns have been below the cost of capital. on average telecommunications and energy concessions have fared much better than water concessions. seven out of 10 water concessions had negative rates of return and two concessions had returns that were lower than the cost of capital of the private companies. private water operators private water operators come in very different forms from multinational corporations to small enterprises. according to the pinsent masons water yearbook 2010–11, 909 million people (13% of the world population) were served by private operators. the largest private water companies are: the french firm veolia environnement (vivendi), serving 125.4 million in 2011; the french firm suez, serving 124.3 million people in 2011 with its us subsidiary united water and its spanish subsidiary aguas de barcelona; the spanish firm fomento de construcciones y contratas sa (fcc), serving 28.2 million people in 2011; the german firm rwe, serving 18.3 million people in 2011; the italian firm acea, serving 18 million people in 2011; the british firm thames water, indirectly owned by macquarie group, an australian investment bank; the french firm saur, serving 12.4 million in 2011; and the us firm american water, serving 16.8 million in 2011. domestic water operators have a strong presence in brazil, colombia, china, malaysia, and the philippines. public water companies also sometimes participate in bids for private water contracts. for example, the moroccan state-owned water utility onep has won a bid in cameroon and the dutch publicly owned water firm vitens has won a management contract in ghana.