but连接词后找不到完整的句型

Even not always these discharges are necessarily waste, but, accidental leaks that occur despite the existing controls in the handling of this type of substances, which can cause adverse environmental impacts on the surrounding environment.

这个句子在but这个连接词后找不到完整的句子呢？哪位老师可以帮忙做下句子的成分剖析吗？but后面开始好了

Even not always these discharges are necessarily waste, but, accidental leaks that occur despite the existing controls in the handling of this type of substances, which can cause adverse environmental impacts on the surrounding environment.

but后省略了they are, accidental leaks是but后分句的表语。逗号提示这里有省略。that引导定语从句修饰leaks，所以that并不多余。

假如删去that, 虽然后面也可以成为一个句子，但没办法讲解but后的逗号。

其实本句not always因为强调而前置了。恢复正常词序则为：

Even these discharges are not always necessarily waste, but accidental leaks that...

即 Sth is not A, but B结构，只是B带了一个定语从句而已。