Independent sentences should be separated by a full stop and/or connected by conjunctions. Otherwise, they become run-on sentences.
In examples 1-4, two independent sentences are separated by a comma instead of a full stop. This is also known as a comma splice.
  

 

WRONG
RIGHT

1.

Past reclamation has narrowed our harbour,
as a result, the pollutants have become harder to wash out and this has led to a significant drop in water quality.

Past reclamation has narrowed our harbour. As a result, the pollutants have become harder to wash out and this has led to a significant drop in water quality.

2.

This harbour lacks facilities, for example, the residential areas and the parks are not included in the design plan.

This harbour lacks facilities. For example, the residential areas and the parks are not included in the design plan.

3.

Take the destruction of Star Ferry Pier in 2006 as an example, at that time, protests and hunger strikes against the demolition of Star Ferry Pier lasted for several days.

Take the destruction of Star Ferry Pier in 2006 as an example. At that time, protests and hunger strikes against the demolition of Star Ferry Pier lasted for several days.

4.

Take water pollution as an example for comparison,
recently, the Hong Kong government has implemented a scheme to collect and treat sewage on both sides of Victoria Harbour.

Take water pollution as an example for comparison. Recently, the Hong Kong government has implemented a scheme to collect and treat sewage on both sides of Victoria Harbour.

 

In examples 5 and 6, two independent clauses are connected by a conjunctive adverb (i.e. “however”). A full stop is also needed to indicate that what follows is an independent sentence. 
 
WRONG
RIGHT

5.

The government destroyed this heritage site so as to make room for a highway and a green belt on reclaimed land between the Hong Kong station and the Hong Kong Convention and Exhibition Centre,
however, this triggered the residents’ anger.

The government destroyed this heritage site so as to make room for a highway and a green belt on reclaimed land between the Hong Kong station and the Hong Kong Convention and Exhibition Centre. However, this triggered the residents’ anger.

6.

There is no doubt that the development of Victoria Harbour is necessary to maintain and improve the above mentioned functions, however, that development is not satisfactory when compared to another renowned harbour, Sydney Harbour.

There is no doubt that the development of Victoria Harbour is necessary to maintain and improve the above mentioned functions. However, that development is not satisfactory when compared to another renowned harbour, Sydney Harbour.

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