Using a mainstream detergent is not detrimental to the waste water system and end environment.
Where Does The Dirty Water Go?
Domestic sewage, i.e. water from indoor drains and toilets in Australian cities and towns, is collected and treated at wastewater treatment plants (WWTP).  This does not include street or storm water drains, which often flow straight to the sea.
What Happens To The Water At The Treatment Plant?
The treatment of sewage or wastewater involves a series of physical, chemical and biological processes to remove solids, organic matter, pathogens, metals and often the added nutrients . Wastewater treatment has four main stages, with advanced treatment being added to the third stage when the wastewater is returned to potable (drinkable) or near potable standard, ensuring it is suitable for eventual reuse or discharge.  After the final stage, the treated wastewater is nearly indistinguishable from drinking quality water of natural origin. 
Each treatment plant has different available technologies, capacity and treatment levels, some do primary, secondary or tertiary treatment, with tertiary being ideal, and the best for the end environment.
The first preference for using tertiary treated water is on vineyards, parks, ovals. The reuse provides environmental and economic benefits to the state. In many states of Australia, reuse where possible is encouraged and in some cases policy. The second preference is discharge to the environment e.g. a river or ocean.
In many (but not all) jurisdictions in Australia, if released into the ocean or a river, it must be at a purity level that won’t be detrimental to the receiving environment. The receiving sea or inland water environment is tested regularly throughout the year by the relevant State Government regulatory authority and the activity is also licensed.
Many areas have seen increases in investment for improved waste water management in recent years. To gain an understanding of what stage the facility undertakes, and where they discharge, research your local water providers current waste water management processes.
In an ideal world disposal of human waste would have no environmental cost. There are varying environmental impacts of all products used. It is necessary to balancing the costs and benefits of each when choosing which to use, without compromising the end result.
Waste Water Treatment Stages
See this page for a more detailed explanation.
History Of Laundry Detergents And Their Environmental Impact
Soaps were the first washing aids, used to wash laundry by hand with a wash board. These original soaps did not degrade in the environment, the residues remained in waterways. By the 1950’s, drains and rivers often carried persistent mounds of foam, and the water became toxic to small organisms. 
Detergents then replaced soaps but were found to be poorly biodegradable.  Manufacturers subsequently began making washing powders biodegradable, so that they decomposed naturally as soon as possible after use. ‘Builders’ were also added, which bind to and remove ions from water (the more ions, the harder the water) and in doing so, soften the water during the washing process. The purpose of water softeners is to make detergent more effective, so that less is required. Reducing water hardness during the wash cycle is a significant factor in the effectiveness of modern detergents.
Historically, phosphates were used as water softeners in detergents. Excess phosphates cause problems in inland waterways, causing eutrophication, which is the enrichment of bodies of fresh water by inorganic plant nutrients such as nitrate and phosphorus . The consequence can be blue-green algae outbreaks which are toxic to the river ecosystem and also to humans. An example of eutrophication is the depletion of oxygen in the water. Another consequence of eutrophication is the prolific growth of particular plants (including algae) and depletion of light and oxygen in the water, resulting in die off or “dead zones”. With decreased use of phosphates, the negative environmental impact of detergent use has greatly reduced. Phosphates were phased out of Australian detergents by 2008.
Modern Detergents And Their Environmental Impact
You will see modern laundry detergents labelled as ‘biodegradable’. The term biodegradable refers to the ability of a material to be broken down, by a group of biological organisms called decomposers. Decomposers are a necessary component of a balanced ecosystem, present in natural waters and sediments, and are encouraged in sewage treatment works. Bacteria are the most common decomposers.
The Australian Standard for biodegradability (AS1792—Methods to Determine the Biodegradability of Surfactants) requires 80% of the detergent mixture to be degraded within 21 days if the product carries the label ‘biodegradable’. This standard is voluntary,  and as with the removal of phosphates has been taken up by companies in response to consumer demand, applying to both ‘eco’ and ‘non-eco’ detergents. Detergent companies provide this information on their packaging and websites, most are happy to provided information on request.
Surfactants (which are the cleaning agents) in detergent are biodegradable. Through the natural process of chemistry and aided by the waste water treatment process, they are broken down to carbon dioxide (CO₂) and water (H₂O). Only a very small fraction of surfactants are unable to aerobically break down during processes of modern treatment plants. This fraction, due to the predominantly hydrophobic (water repelling) nature of surfactants, adheres to a byproduct ‘sludge’ created during at the treatment plant. The sludge is then further treated, see below. 
The Impact Of Other Laundry Products, e.g. Bleach and Canesten
Pollutants from household products are almost completely eliminated at the treatment plant, the wastewater is treated to a purity level and returned to potable (drinkable) or near potable standard, so that is not detrimental to the receiving environment. Insoluble salts or low water soluble surfactants are processed into a ‘sludge’, which is then decomposed. Once decomposed the sludge is dried, treated and disposed of to landfill. The high content of organic material and nutrients means it can also be used for agricultural purposes as a fertiliser. The very small amount of remaining compounds that have not completely degraded during the treatment process are further degraded in soils.
Bleach (Sodium Hypochlorite) degrades to salt, water and oxygen. A large percentage has already completely degraded by the time it reaches the treatment plant. The biodegradation rate of Benzalkonium chloride, the active ingredient in Canesten, is very slow.  It remains in the sludge at the treatment plant. Although Benzalkonium chloride does biodegrade eventually, when comparing the two sanitisation methods of either bleach or Canesten, overall bleach is a better choice for the environment.
 Asano, T (1998). Wastewater reclamation and reuse. Water Quality Management Series Volume 10, CRC Press, Boca Raton, FL, USA.
 The Disposal of Soaps and Detergents EPA 547/04—April 2004
Author A. Michailov