Throughout the paper let S be a commutative semigroup with identity element 1S. A congruence on S is an equivalence relation Δ such that if Δ relates a and b (written aΔb), then for every c ∈ S we also have acΔbc. Two distinguished
congruences are present in every semigroup: i, the identity relation, and u, the universal relation. They are defined by: aib ⇐⇒ a = b , aub ⇐⇒ a,b ∈ S.
The intersection of two congruences α and β is written as α∩β. A congruence
Δ is said to be essential if for every congruence α = i we have α ∩ Δ = i.
Clearly u is an essential congruence. We say Δ is proper if Δ = u.
Let D be the class of commutative semigroups with an identity element 1 and having no proper essential congruences. Oehmke [3], proved that if S ∈ D,
then every ideal of S is generated by an idempotent and proved that the set of ideals of S is linearly ordered by inclusion. Now we shall prove that if S ∈ D,