capture both rainwater runoff and the sewage produced by four million people. As a failsafe, to prevent sewage backing up and flooding people's homes, Bazalgette's system had the ability to overflow into the Thames via 57 combined sewer overflows (CSO) along the banks of the river. In 2001, London's population density was 18,457 people per square kilometre, compared to just 6,825 per square kilometre in Bazalgette's day. As the population has grown, so has development involving the building on and paving over large areas. This has altered the natural drainage of the sewerage catchment so that now most rainfall and surface water run-off goes directly into London's sewers, rather than being naturally absorbed into permeable ground. An area twice the size of Hyde Park has been lost to hard surfacing every year up to 2001. In the 2011 census returns (that is, omitting residents who did not disclose their residence) Greater London hosted just over 8.1 million residents and Thames Water in a leaflet of that year explained how its works and how sewers needed expansion. The older inner boroughs have combined sewerage systems which cannot cope with the extra hard standings. As London continued to grow the new developments and suburbs were equipped with separated foul water and rainwater drainage pipe work. Extending such separate systems would stop sewage flooding of homes and businesses, as well as into the rivers and the estuary, would reduce carbon emissions, and be cheaper in the long term, than the multitude of pumped water tanks otherwise needed to stop homes flooding plus the Tideway tunnel. Pipes would be laid in a number of streets in each Borough each year over say twenty years, limiting traffic diversions. By 2011 overflows into the river took place more than once a week and as little as 2mm of rainfall can trigger a discharge. The mean total of this was , that is 39 million tonnes, of storm sewage entering the river. These discharges, a mixture of raw sewage and rainwater, needed to be cut to comply with the EU's Waste Water Treatment Directive (UWWTD). If cut to such levels, it would thus restore the ecology of the river to that of 1865–70 or better, that is, before such unmitigated growth took place. Options assessment Instigated in 2001, the Thames Tideway Strategic Study, conducted by a group comprising Thames Water, the Environment Agency, DEFRA and the Greater